<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with Wdesk from Workiva-->
<!--Copyright 2021 Workiva-->
<!--r:aa9e2f98-4963-4ceb-9110-1edadc67762c,g:4d7b3069-a1d5-4982-8896-d9fa31345084-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://arcutis.com/role/CondensedStatementsofConvertiblePreferredStockandStockholdersEquityDeficit" xlink:type="simple" xlink:href="arqt-20210930.xsd#CondensedStatementsofConvertiblePreferredStockandStockholdersEquityDeficit"/>
  <link:definitionLink xlink:role="http://arcutis.com/role/CondensedStatementsofConvertiblePreferredStockandStockholdersEquityDeficit" xlink:type="extended" id="i7b45d2c72cfa4b349ffcb7d6bedad97c_CondensedStatementsofConvertiblePreferredStockandStockholdersEquityDeficit">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_5388d318-7373-4d8a-9c59-93a7062f5f46" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_3bfa0b2a-031c-4641-9088-b7b05e2cf18b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncreaseDecreaseInTemporaryEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_5388d318-7373-4d8a-9c59-93a7062f5f46" xlink:to="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_3bfa0b2a-031c-4641-9088-b7b05e2cf18b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquitySharesOutstanding_a0bc3af6-659f-4ab3-926c-e0c5b5411919" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TemporaryEquitySharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_3bfa0b2a-031c-4641-9088-b7b05e2cf18b" xlink:to="loc_us-gaap_TemporaryEquitySharesOutstanding_a0bc3af6-659f-4ab3-926c-e0c5b5411919" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_623813bd-a4a8-466b-895a-d3ff891db63f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TemporaryEquityCarryingAmountAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_3bfa0b2a-031c-4641-9088-b7b05e2cf18b" xlink:to="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_623813bd-a4a8-466b-895a-d3ff891db63f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_TemporaryEquitySharesConversionOfConvertibleSecurities_e19de2d2-e5c8-42bf-be54-cc673313534f" xlink:href="arqt-20210930.xsd#arqt_TemporaryEquitySharesConversionOfConvertibleSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_3bfa0b2a-031c-4641-9088-b7b05e2cf18b" xlink:to="loc_arqt_TemporaryEquitySharesConversionOfConvertibleSecurities_e19de2d2-e5c8-42bf-be54-cc673313534f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_TemporaryEquityValueConversionOfConvertibleSecurities_bdd3207a-f40a-4f04-9708-6ef4ff1dd6de" xlink:href="arqt-20210930.xsd#arqt_TemporaryEquityValueConversionOfConvertibleSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInTemporaryEquityRollForward_3bfa0b2a-031c-4641-9088-b7b05e2cf18b" xlink:to="loc_arqt_TemporaryEquityValueConversionOfConvertibleSecurities_bdd3207a-f40a-4f04-9708-6ef4ff1dd6de" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquitySharesOutstanding_6ef913f8-258c-4f75-9703-ad276f6f20cd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TemporaryEquitySharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityCarryingAmountAttributableToParent_8605eae5-253c-4208-84bc-13eeaab71b38" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TemporaryEquityCarryingAmountAttributableToParent"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f270977f-18fc-4da8-90b4-2f5b8751c4be" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_5388d318-7373-4d8a-9c59-93a7062f5f46" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f270977f-18fc-4da8-90b4-2f5b8751c4be" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesOutstanding_9ce7f554-b240-4253-bce9-efc16e7a0290" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f270977f-18fc-4da8-90b4-2f5b8751c4be" xlink:to="loc_us-gaap_SharesOutstanding_9ce7f554-b240-4253-bce9-efc16e7a0290" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_49532a8c-8543-4a3f-8593-920870bdb239" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f270977f-18fc-4da8-90b4-2f5b8751c4be" xlink:to="loc_us-gaap_StockholdersEquity_49532a8c-8543-4a3f-8593-920870bdb239" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_02be046c-69c8-4c63-bd2b-4853909d2c65" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f270977f-18fc-4da8-90b4-2f5b8751c4be" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_02be046c-69c8-4c63-bd2b-4853909d2c65" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities_64bda19c-8c10-407d-ae46-009e55b5f313" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f270977f-18fc-4da8-90b4-2f5b8751c4be" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities_64bda19c-8c10-407d-ae46-009e55b5f313" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_6ceb9e2f-b462-4ced-9a32-5dd1e50c42a3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f270977f-18fc-4da8-90b4-2f5b8751c4be" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_6ceb9e2f-b462-4ced-9a32-5dd1e50c42a3" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_dd5fbf07-47f4-4152-9970-dea0bca019bf" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f270977f-18fc-4da8-90b4-2f5b8751c4be" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_dd5fbf07-47f4-4152-9970-dea0bca019bf" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_ebbbb0f8-6891-4d4d-b177-c15875d3560b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f270977f-18fc-4da8-90b4-2f5b8751c4be" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_ebbbb0f8-6891-4d4d-b177-c15875d3560b" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_2a53f43c-39ac-41a8-ab00-982f574588ff" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f270977f-18fc-4da8-90b4-2f5b8751c4be" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_2a53f43c-39ac-41a8-ab00-982f574588ff" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross_960f8a86-27b8-449d-9389-c84376c16b91" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f270977f-18fc-4da8-90b4-2f5b8751c4be" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross_960f8a86-27b8-449d-9389-c84376c16b91" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_StockIssuedDuringPeriodVestingOfFounderSharesSubjectToRepurchaseShares_36128dc1-4e59-4692-b8b2-056284db717d" xlink:href="arqt-20210930.xsd#arqt_StockIssuedDuringPeriodVestingOfFounderSharesSubjectToRepurchaseShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f270977f-18fc-4da8-90b4-2f5b8751c4be" xlink:to="loc_arqt_StockIssuedDuringPeriodVestingOfFounderSharesSubjectToRepurchaseShares_36128dc1-4e59-4692-b8b2-056284db717d" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_StockIssuedDuringPeriodLapseOfRepurchaseRightsRelatedToCommonStockIssuedPursuantToEarlyExercisesShares_142b77eb-e7fb-4bd3-b034-17f7abea1cc7" xlink:href="arqt-20210930.xsd#arqt_StockIssuedDuringPeriodLapseOfRepurchaseRightsRelatedToCommonStockIssuedPursuantToEarlyExercisesShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f270977f-18fc-4da8-90b4-2f5b8751c4be" xlink:to="loc_arqt_StockIssuedDuringPeriodLapseOfRepurchaseRightsRelatedToCommonStockIssuedPursuantToEarlyExercisesShares_142b77eb-e7fb-4bd3-b034-17f7abea1cc7" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_StockIssuedDuringPeriodLapseOfRepurchaseRightsPursuantToEarlyStockExercises_bf954de8-4a5c-4a7b-9ff1-bf7e1e4199c3" xlink:href="arqt-20210930.xsd#arqt_StockIssuedDuringPeriodLapseOfRepurchaseRightsPursuantToEarlyStockExercises"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f270977f-18fc-4da8-90b4-2f5b8751c4be" xlink:to="loc_arqt_StockIssuedDuringPeriodLapseOfRepurchaseRightsPursuantToEarlyStockExercises_bf954de8-4a5c-4a7b-9ff1-bf7e1e4199c3" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_ced6527f-32d6-4e4b-b4e1-8c23783b8466" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f270977f-18fc-4da8-90b4-2f5b8751c4be" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_ced6527f-32d6-4e4b-b4e1-8c23783b8466" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_05de3131-dae2-45d6-852c-21287a231102" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f270977f-18fc-4da8-90b4-2f5b8751c4be" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_05de3131-dae2-45d6-852c-21287a231102" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_42e6d4e7-f4f5-4ec1-8745-e3c85ad62829" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f270977f-18fc-4da8-90b4-2f5b8751c4be" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_42e6d4e7-f4f5-4ec1-8745-e3c85ad62829" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_03a75a9a-3398-41bc-a092-c6e4a6ad0d75" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f270977f-18fc-4da8-90b4-2f5b8751c4be" xlink:to="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_03a75a9a-3398-41bc-a092-c6e4a6ad0d75" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_683dbb65-1260-4f66-8ea4-e272f7a07833" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_f270977f-18fc-4da8-90b4-2f5b8751c4be" xlink:to="loc_us-gaap_NetIncomeLoss_683dbb65-1260-4f66-8ea4-e272f7a07833" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_803f8b6d-895a-44a7-ba68-494376c43a77" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockholdersEquity"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesOutstanding_2e048437-c54d-4025-a505-28d231403d06" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_4758be5f-be05-423d-9f11-a035adadba12" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_5388d318-7373-4d8a-9c59-93a7062f5f46" xlink:to="loc_us-gaap_StatementTable_4758be5f-be05-423d-9f11-a035adadba12" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_d5f13e3c-42ea-4721-91ef-960883f91f71" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_4758be5f-be05-423d-9f11-a035adadba12" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_d5f13e3c-42ea-4721-91ef-960883f91f71" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_d5f13e3c-42ea-4721-91ef-960883f91f71_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_d5f13e3c-42ea-4721-91ef-960883f91f71" xlink:to="loc_us-gaap_EquityComponentDomain_d5f13e3c-42ea-4721-91ef-960883f91f71_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_f0270a4b-c2b3-48b5-80a8-a8b8eca7134f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_d5f13e3c-42ea-4721-91ef-960883f91f71" xlink:to="loc_us-gaap_EquityComponentDomain_f0270a4b-c2b3-48b5-80a8-a8b8eca7134f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_266f1e1f-72b2-4c40-9f1f-a607cd56fc6a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_f0270a4b-c2b3-48b5-80a8-a8b8eca7134f" xlink:to="loc_us-gaap_CommonStockMember_266f1e1f-72b2-4c40-9f1f-a607cd56fc6a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_2f02e63b-4efc-4e13-be77-4e5bb58bdf15" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_f0270a4b-c2b3-48b5-80a8-a8b8eca7134f" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_2f02e63b-4efc-4e13-be77-4e5bb58bdf15" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_0ff8891d-8a8e-4ff4-a9fd-0ac85bc1dde7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_f0270a4b-c2b3-48b5-80a8-a8b8eca7134f" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_0ff8891d-8a8e-4ff4-a9fd-0ac85bc1dde7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_d89ffe01-04ec-4340-a69f-1a28d2ba8441" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_f0270a4b-c2b3-48b5-80a8-a8b8eca7134f" xlink:to="loc_us-gaap_RetainedEarningsMember_d89ffe01-04ec-4340-a69f-1a28d2ba8441" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://arcutis.com/role/CondensedStatementsofConvertiblePreferredStockandStockholdersEquityDeficitParenthetical" xlink:type="simple" xlink:href="arqt-20210930.xsd#CondensedStatementsofConvertiblePreferredStockandStockholdersEquityDeficitParenthetical"/>
  <link:definitionLink xlink:role="http://arcutis.com/role/CondensedStatementsofConvertiblePreferredStockandStockholdersEquityDeficitParenthetical" xlink:type="extended" id="i731e73cd20c94a4db46745d409a966fc_CondensedStatementsofConvertiblePreferredStockandStockholdersEquityDeficitParenthetical">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_7f29be31-7166-46e2-b3fe-ff98ef0f53ef" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfStockIssuanceCosts_5fd317b9-52f5-4735-861f-398553602832" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PaymentsOfStockIssuanceCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_7f29be31-7166-46e2-b3fe-ff98ef0f53ef" xlink:to="loc_us-gaap_PaymentsOfStockIssuanceCosts_5fd317b9-52f5-4735-861f-398553602832" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_95372fcd-c9af-495a-bf09-a15a81b4fd2b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_7f29be31-7166-46e2-b3fe-ff98ef0f53ef" xlink:to="loc_us-gaap_StatementTable_95372fcd-c9af-495a-bf09-a15a81b4fd2b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_a26e7c94-21ba-4d81-84cb-4a482f060eb5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_95372fcd-c9af-495a-bf09-a15a81b4fd2b" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_a26e7c94-21ba-4d81-84cb-4a482f060eb5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_a26e7c94-21ba-4d81-84cb-4a482f060eb5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_a26e7c94-21ba-4d81-84cb-4a482f060eb5" xlink:to="loc_us-gaap_EquityComponentDomain_a26e7c94-21ba-4d81-84cb-4a482f060eb5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_2044265d-ce16-466f-bcf0-fd8facdf7514" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_a26e7c94-21ba-4d81-84cb-4a482f060eb5" xlink:to="loc_us-gaap_EquityComponentDomain_2044265d-ce16-466f-bcf0-fd8facdf7514" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_9d33d26a-fc10-4a2a-9337-183e00b30432" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_2044265d-ce16-466f-bcf0-fd8facdf7514" xlink:to="loc_us-gaap_CommonStockMember_9d33d26a-fc10-4a2a-9337-183e00b30432" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://arcutis.com/role/OrganizationandDescriptionofBusinessDetails" xlink:type="simple" xlink:href="arqt-20210930.xsd#OrganizationandDescriptionofBusinessDetails"/>
  <link:definitionLink xlink:role="http://arcutis.com/role/OrganizationandDescriptionofBusinessDetails" xlink:type="extended" id="i754185a5128a4e7594fb7087d5a3c641_OrganizationandDescriptionofBusinessDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockLineItems_d21ff8bd-5025-41c1-b8c1-c5dacaa5a198" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsidiarySaleOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityNoteStockSplitConversionRatio1_81385eca-03f0-4b1e-862a-f2ad1fe773b4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockholdersEquityNoteStockSplitConversionRatio1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_d21ff8bd-5025-41c1-b8c1-c5dacaa5a198" xlink:to="loc_us-gaap_StockholdersEquityNoteStockSplitConversionRatio1_81385eca-03f0-4b1e-862a-f2ad1fe773b4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_bffb0d1e-6720-4728-a529-27a29391612a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_d21ff8bd-5025-41c1-b8c1-c5dacaa5a198" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_bffb0d1e-6720-4728-a529-27a29391612a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_c0709187-1c18-48b7-8393-4bc15edff82f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SaleOfStockPricePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_d21ff8bd-5025-41c1-b8c1-c5dacaa5a198" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_c0709187-1c18-48b7-8393-4bc15edff82f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceInitialPublicOffering_ab2457b0-0a4e-444d-b6d5-d518082101ea" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ProceedsFromIssuanceInitialPublicOffering"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_d21ff8bd-5025-41c1-b8c1-c5dacaa5a198" xlink:to="loc_us-gaap_ProceedsFromIssuanceInitialPublicOffering_ab2457b0-0a4e-444d-b6d5-d518082101ea" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquitySharesOutstanding_61dfc45d-d2e0-4638-84b7-aceedb625d8c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TemporaryEquitySharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_d21ff8bd-5025-41c1-b8c1-c5dacaa5a198" xlink:to="loc_us-gaap_TemporaryEquitySharesOutstanding_61dfc45d-d2e0-4638-84b7-aceedb625d8c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_02721667-1f01-4f33-98bb-d51c25a87d45" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_d21ff8bd-5025-41c1-b8c1-c5dacaa5a198" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_02721667-1f01-4f33-98bb-d51c25a87d45" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_SaleOfStockAuthorizedConsiderationOnTransaction_b5f09bae-87a4-42eb-870a-f3fca070a152" xlink:href="arqt-20210930.xsd#arqt_SaleOfStockAuthorizedConsiderationOnTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_d21ff8bd-5025-41c1-b8c1-c5dacaa5a198" xlink:to="loc_arqt_SaleOfStockAuthorizedConsiderationOnTransaction_b5f09bae-87a4-42eb-870a-f3fca070a152" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_SaleOfStockAgentFeePercentageOfGrossProceeds_e475f90f-f16c-49a2-8087-2664da7d93b8" xlink:href="arqt-20210930.xsd#arqt_SaleOfStockAgentFeePercentageOfGrossProceeds"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_d21ff8bd-5025-41c1-b8c1-c5dacaa5a198" xlink:to="loc_arqt_SaleOfStockAgentFeePercentageOfGrossProceeds_e475f90f-f16c-49a2-8087-2664da7d93b8" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_7ee015b8-c86b-4605-a9c3-02c7a66ef13d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetainedEarningsAccumulatedDeficit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_d21ff8bd-5025-41c1-b8c1-c5dacaa5a198" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_7ee015b8-c86b-4605-a9c3-02c7a66ef13d" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_CashAndCashEquivalentsRestrictedCashAndCashEquivalentsAndShortTermInvestments_fdf8196c-dff4-4a6d-9fc1-72e8aaaa8f67" xlink:href="arqt-20210930.xsd#arqt_CashAndCashEquivalentsRestrictedCashAndCashEquivalentsAndShortTermInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_d21ff8bd-5025-41c1-b8c1-c5dacaa5a198" xlink:to="loc_arqt_CashAndCashEquivalentsRestrictedCashAndCashEquivalentsAndShortTermInvestments_fdf8196c-dff4-4a6d-9fc1-72e8aaaa8f67" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiaryOrEquityMethodInvesteeSaleOfStockBySubsidiaryOrEquityInvesteeTable_16ebef9e-b3a3-4a68-a89d-0a1a06b9e685" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsidiaryOrEquityMethodInvesteeSaleOfStockBySubsidiaryOrEquityInvesteeTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_d21ff8bd-5025-41c1-b8c1-c5dacaa5a198" xlink:to="loc_us-gaap_SubsidiaryOrEquityMethodInvesteeSaleOfStockBySubsidiaryOrEquityInvesteeTable_16ebef9e-b3a3-4a68-a89d-0a1a06b9e685" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_cacf901f-2a12-4cc0-ba2e-fba68ed6aab0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsidiaryOrEquityMethodInvesteeSaleOfStockBySubsidiaryOrEquityInvesteeTable_16ebef9e-b3a3-4a68-a89d-0a1a06b9e685" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_cacf901f-2a12-4cc0-ba2e-fba68ed6aab0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_cacf901f-2a12-4cc0-ba2e-fba68ed6aab0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_cacf901f-2a12-4cc0-ba2e-fba68ed6aab0" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_cacf901f-2a12-4cc0-ba2e-fba68ed6aab0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_9d4da5f1-56e6-4a2e-98b8-80b5561ce3cf" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_cacf901f-2a12-4cc0-ba2e-fba68ed6aab0" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_9d4da5f1-56e6-4a2e-98b8-80b5561ce3cf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IPOMember_45c1fc50-6ba9-4235-8b07-01d0bad0be5c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IPOMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_9d4da5f1-56e6-4a2e-98b8-80b5561ce3cf" xlink:to="loc_us-gaap_IPOMember_45c1fc50-6ba9-4235-8b07-01d0bad0be5c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OverAllotmentOptionMember_98d19cdb-bee0-4027-b9fc-28413380f777" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OverAllotmentOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_9d4da5f1-56e6-4a2e-98b8-80b5561ce3cf" xlink:to="loc_us-gaap_OverAllotmentOptionMember_98d19cdb-bee0-4027-b9fc-28413380f777" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_SecondaryEquityPublicOfferingMember_799331a7-2f54-4ba5-9da7-68ef4a04c999" xlink:href="arqt-20210930.xsd#arqt_SecondaryEquityPublicOfferingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_9d4da5f1-56e6-4a2e-98b8-80b5561ce3cf" xlink:to="loc_arqt_SecondaryEquityPublicOfferingMember_799331a7-2f54-4ba5-9da7-68ef4a04c999" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrivatePlacementMember_83e40d32-dffd-4710-ba8d-5daa508b8922" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PrivatePlacementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_9d4da5f1-56e6-4a2e-98b8-80b5561ce3cf" xlink:to="loc_us-gaap_PrivatePlacementMember_83e40d32-dffd-4710-ba8d-5daa508b8922" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_ThirdEquityPublicOfferingMember_b1283fe6-33cd-4e46-b804-481e675a7ca3" xlink:href="arqt-20210930.xsd#arqt_ThirdEquityPublicOfferingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_9d4da5f1-56e6-4a2e-98b8-80b5561ce3cf" xlink:to="loc_arqt_ThirdEquityPublicOfferingMember_b1283fe6-33cd-4e46-b804-481e675a7ca3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_AtTheMarketMember_5e2169a6-4de4-41e8-a6a9-994e6d5c7e35" xlink:href="arqt-20210930.xsd#arqt_AtTheMarketMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_9d4da5f1-56e6-4a2e-98b8-80b5561ce3cf" xlink:to="loc_arqt_AtTheMarketMember_5e2169a6-4de4-41e8-a6a9-994e6d5c7e35" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_ffb33083-761e-4d18-a76f-c4b72627215e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsidiaryOrEquityMethodInvesteeSaleOfStockBySubsidiaryOrEquityInvesteeTable_16ebef9e-b3a3-4a68-a89d-0a1a06b9e685" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_ffb33083-761e-4d18-a76f-c4b72627215e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_ffb33083-761e-4d18-a76f-c4b72627215e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_ffb33083-761e-4d18-a76f-c4b72627215e" xlink:to="loc_us-gaap_EquityComponentDomain_ffb33083-761e-4d18-a76f-c4b72627215e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_613d1bc8-aa29-478d-8a24-4d00363fd6f8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_ffb33083-761e-4d18-a76f-c4b72627215e" xlink:to="loc_us-gaap_EquityComponentDomain_613d1bc8-aa29-478d-8a24-4d00363fd6f8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_d625dbaa-a936-4f6d-939a-572c754251b6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_613d1bc8-aa29-478d-8a24-4d00363fd6f8" xlink:to="loc_us-gaap_CommonStockMember_d625dbaa-a936-4f6d-939a-572c754251b6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_154800bd-6b2a-40fe-9a11-15abdb2229b5" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsidiaryOrEquityMethodInvesteeSaleOfStockBySubsidiaryOrEquityInvesteeTable_16ebef9e-b3a3-4a68-a89d-0a1a06b9e685" xlink:to="loc_srt_CounterpartyNameAxis_154800bd-6b2a-40fe-9a11-15abdb2229b5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_154800bd-6b2a-40fe-9a11-15abdb2229b5_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_154800bd-6b2a-40fe-9a11-15abdb2229b5" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_154800bd-6b2a-40fe-9a11-15abdb2229b5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_74e2ecef-6f0e-4841-b82a-35b87d63e310" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_154800bd-6b2a-40fe-9a11-15abdb2229b5" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_74e2ecef-6f0e-4841-b82a-35b87d63e310" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_CowenAndCompanyLLCMember_49e614ae-b51c-4c81-bd3c-359bff5905ed" xlink:href="arqt-20210930.xsd#arqt_CowenAndCompanyLLCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_74e2ecef-6f0e-4841-b82a-35b87d63e310" xlink:to="loc_arqt_CowenAndCompanyLLCMember_49e614ae-b51c-4c81-bd3c-359bff5905ed" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_b709155f-3e10-4ba4-ab38-cc3602587795" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsidiaryOrEquityMethodInvesteeSaleOfStockBySubsidiaryOrEquityInvesteeTable_16ebef9e-b3a3-4a68-a89d-0a1a06b9e685" xlink:to="loc_us-gaap_StatementClassOfStockAxis_b709155f-3e10-4ba4-ab38-cc3602587795" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_b709155f-3e10-4ba4-ab38-cc3602587795_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_b709155f-3e10-4ba4-ab38-cc3602587795" xlink:to="loc_us-gaap_ClassOfStockDomain_b709155f-3e10-4ba4-ab38-cc3602587795_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_ffb19856-43af-43c7-a0f9-99502ad90344" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_b709155f-3e10-4ba4-ab38-cc3602587795" xlink:to="loc_us-gaap_ClassOfStockDomain_ffb19856-43af-43c7-a0f9-99502ad90344" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_295198d9-1982-4023-9357-a26dcf860156" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_ffb19856-43af-43c7-a0f9-99502ad90344" xlink:to="loc_us-gaap_CommonStockMember_295198d9-1982-4023-9357-a26dcf860156" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://arcutis.com/role/SummaryofSignificantAccountingPoliciesDetails" xlink:type="simple" xlink:href="arqt-20210930.xsd#SummaryofSignificantAccountingPoliciesDetails"/>
  <link:definitionLink xlink:role="http://arcutis.com/role/SummaryofSignificantAccountingPoliciesDetails" xlink:type="extended" id="i6034c92757fd4151a4e1536b28aa09fc_SummaryofSignificantAccountingPoliciesDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_b0520d70-0514-497b-ac67-98e7752ab421" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfOperatingSegments_d624ae07-b142-4a20-ab56-752272928a72" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NumberOfOperatingSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_b0520d70-0514-497b-ac67-98e7752ab421" xlink:to="loc_us-gaap_NumberOfOperatingSegments_d624ae07-b142-4a20-ab56-752272928a72" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalents_d2a09f5d-e665-4533-b9a6-c744fa23e39c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_b0520d70-0514-497b-ac67-98e7752ab421" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalents_d2a09f5d-e665-4533-b9a6-c744fa23e39c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLoss_e9bdc4fd-6b4a-4e1b-9d86-e0c6549b80fa" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_b0520d70-0514-497b-ac67-98e7752ab421" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLoss_e9bdc4fd-6b4a-4e1b-9d86-e0c6549b80fa" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_c1e9f541-af9d-4b40-a055-4450a6113743" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_b0520d70-0514-497b-ac67-98e7752ab421" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_c1e9f541-af9d-4b40-a055-4450a6113743" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TangibleAssetImpairmentCharges_9754c581-ffcd-41d9-b75f-314de601be6d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TangibleAssetImpairmentCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_b0520d70-0514-497b-ac67-98e7752ab421" xlink:to="loc_us-gaap_TangibleAssetImpairmentCharges_9754c581-ffcd-41d9-b75f-314de601be6d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_016972b7-089d-4f66-9f93-355236ecc7e2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_b0520d70-0514-497b-ac67-98e7752ab421" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_016972b7-089d-4f66-9f93-355236ecc7e2" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense_885b9d81-b136-494c-858c-1c5772a37015" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_b0520d70-0514-497b-ac67-98e7752ab421" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense_885b9d81-b136-494c-858c-1c5772a37015" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_64b48825-2772-427f-a8c1-352e4739cf61" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_b0520d70-0514-497b-ac67-98e7752ab421" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_64b48825-2772-427f-a8c1-352e4739cf61" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_393e36b6-f63d-46e0-b8c2-8839f926675a" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_64b48825-2772-427f-a8c1-352e4739cf61" xlink:to="loc_srt_RangeAxis_393e36b6-f63d-46e0-b8c2-8839f926675a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_393e36b6-f63d-46e0-b8c2-8839f926675a_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_393e36b6-f63d-46e0-b8c2-8839f926675a" xlink:to="loc_srt_RangeMember_393e36b6-f63d-46e0-b8c2-8839f926675a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_87d5a7af-8525-4c0c-89d2-60629d2f0b1b" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_393e36b6-f63d-46e0-b8c2-8839f926675a" xlink:to="loc_srt_RangeMember_87d5a7af-8525-4c0c-89d2-60629d2f0b1b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_b1e645b2-905d-475f-aeaf-01e90c9e5ed8" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_87d5a7af-8525-4c0c-89d2-60629d2f0b1b" xlink:to="loc_srt_MinimumMember_b1e645b2-905d-475f-aeaf-01e90c9e5ed8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_b9c96e00-9a97-4fdb-a0a3-ce2ae44dbdf8" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_87d5a7af-8525-4c0c-89d2-60629d2f0b1b" xlink:to="loc_srt_MaximumMember_b9c96e00-9a97-4fdb-a0a3-ce2ae44dbdf8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_d25ff025-3314-408e-9ee3-4488a8cc0ba4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_64b48825-2772-427f-a8c1-352e4739cf61" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_d25ff025-3314-408e-9ee3-4488a8cc0ba4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_d25ff025-3314-408e-9ee3-4488a8cc0ba4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_d25ff025-3314-408e-9ee3-4488a8cc0ba4" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_d25ff025-3314-408e-9ee3-4488a8cc0ba4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_fa1673c2-bdaa-4a5d-bc99-accea43b5c5b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_d25ff025-3314-408e-9ee3-4488a8cc0ba4" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_fa1673c2-bdaa-4a5d-bc99-accea43b5c5b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_ThirdEquityPublicOfferingMember_fbfab637-999b-4db7-a03e-a20cab4e2d67" xlink:href="arqt-20210930.xsd#arqt_ThirdEquityPublicOfferingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_fa1673c2-bdaa-4a5d-bc99-accea43b5c5b" xlink:to="loc_arqt_ThirdEquityPublicOfferingMember_fbfab637-999b-4db7-a03e-a20cab4e2d67" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OverAllotmentOptionMember_2df6720f-07bd-434f-8131-def0f92a1c1d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OverAllotmentOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_fa1673c2-bdaa-4a5d-bc99-accea43b5c5b" xlink:to="loc_us-gaap_OverAllotmentOptionMember_2df6720f-07bd-434f-8131-def0f92a1c1d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_6cc4c72d-54e1-494c-b990-39ba1cfc7b44" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_64b48825-2772-427f-a8c1-352e4739cf61" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_6cc4c72d-54e1-494c-b990-39ba1cfc7b44" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_6cc4c72d-54e1-494c-b990-39ba1cfc7b44_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_6cc4c72d-54e1-494c-b990-39ba1cfc7b44" xlink:to="loc_us-gaap_EquityComponentDomain_6cc4c72d-54e1-494c-b990-39ba1cfc7b44_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_c4a646a2-42c1-4018-88e1-686389c6d8c7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_6cc4c72d-54e1-494c-b990-39ba1cfc7b44" xlink:to="loc_us-gaap_EquityComponentDomain_c4a646a2-42c1-4018-88e1-686389c6d8c7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_8a15558f-a7b7-4794-b979-a2e6cd0da3e0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_c4a646a2-42c1-4018-88e1-686389c6d8c7" xlink:to="loc_us-gaap_CommonStockMember_8a15558f-a7b7-4794-b979-a2e6cd0da3e0" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://arcutis.com/role/FairValueMeasurementsFairValueofAssetsMeasuredonaRecurringBasisDetails" xlink:type="simple" xlink:href="arqt-20210930.xsd#FairValueMeasurementsFairValueofAssetsMeasuredonaRecurringBasisDetails"/>
  <link:definitionLink xlink:role="http://arcutis.com/role/FairValueMeasurementsFairValueofAssetsMeasuredonaRecurringBasisDetails" xlink:type="extended" id="iefe1a7a886aa4819b3a5c9c704121cc3_FairValueMeasurementsFairValueofAssetsMeasuredonaRecurringBasisDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_e569d100-3db7-417e-9813-6ed3e643a280" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_30dbf8b0-c98f-4e08-9561-d15c43147a94" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_e569d100-3db7-417e-9813-6ed3e643a280" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_30dbf8b0-c98f-4e08-9561-d15c43147a94" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_CashAndCashEquivalentsAndDebtSecuritiesAvailableForSaleFairValueDisclosure_52370add-ea1b-471d-b61d-f75758e684fc" xlink:href="arqt-20210930.xsd#arqt_CashAndCashEquivalentsAndDebtSecuritiesAvailableForSaleFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_e569d100-3db7-417e-9813-6ed3e643a280" xlink:to="loc_arqt_CashAndCashEquivalentsAndDebtSecuritiesAvailableForSaleFairValueDisclosure_52370add-ea1b-471d-b61d-f75758e684fc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_f31b3e17-30a4-4cd4-b826-7ad6a0a6e3a4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_e569d100-3db7-417e-9813-6ed3e643a280" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_f31b3e17-30a4-4cd4-b826-7ad6a0a6e3a4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_82b28c1c-509a-418b-ab06-37e860f2f7e4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_e569d100-3db7-417e-9813-6ed3e643a280" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_82b28c1c-509a-418b-ab06-37e860f2f7e4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_d33e687f-8f2e-488c-ae19-439074f579d8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_82b28c1c-509a-418b-ab06-37e860f2f7e4" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_d33e687f-8f2e-488c-ae19-439074f579d8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_d33e687f-8f2e-488c-ae19-439074f579d8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_d33e687f-8f2e-488c-ae19-439074f579d8" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_d33e687f-8f2e-488c-ae19-439074f579d8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_109913ce-f092-451b-bd88-61ec916f76fd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_d33e687f-8f2e-488c-ae19-439074f579d8" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_109913ce-f092-451b-bd88-61ec916f76fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_ec62dc01-eb3a-4401-8ce2-6b8a1154c843" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_109913ce-f092-451b-bd88-61ec916f76fd" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_ec62dc01-eb3a-4401-8ce2-6b8a1154c843" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_cc0fadb5-e0d3-4ee9-a5ec-34d7816742f8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_109913ce-f092-451b-bd88-61ec916f76fd" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_cc0fadb5-e0d3-4ee9-a5ec-34d7816742f8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_135347a1-7408-45fc-8781-ba31efa862a9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_109913ce-f092-451b-bd88-61ec916f76fd" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_135347a1-7408-45fc-8781-ba31efa862a9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAxis_cb606dcf-2d52-40f8-b36c-deaecd34daaf" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CashAndCashEquivalentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_82b28c1c-509a-418b-ab06-37e860f2f7e4" xlink:to="loc_us-gaap_CashAndCashEquivalentsAxis_cb606dcf-2d52-40f8-b36c-deaecd34daaf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_cb606dcf-2d52-40f8-b36c-deaecd34daaf_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_cb606dcf-2d52-40f8-b36c-deaecd34daaf" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_cb606dcf-2d52-40f8-b36c-deaecd34daaf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_62c1b9e2-4f08-4d8d-91a6-8cabfed6ddc7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_cb606dcf-2d52-40f8-b36c-deaecd34daaf" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_62c1b9e2-4f08-4d8d-91a6-8cabfed6ddc7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsMember_005c4654-0dc5-45ae-ae13-3489030eb555" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_MoneyMarketFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_62c1b9e2-4f08-4d8d-91a6-8cabfed6ddc7" xlink:to="loc_us-gaap_MoneyMarketFundsMember_005c4654-0dc5-45ae-ae13-3489030eb555" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_bae56a3e-b451-4e80-8a33-8dff90194f74" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_82b28c1c-509a-418b-ab06-37e860f2f7e4" xlink:to="loc_us-gaap_FinancialInstrumentAxis_bae56a3e-b451-4e80-8a33-8dff90194f74" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_bae56a3e-b451-4e80-8a33-8dff90194f74_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_bae56a3e-b451-4e80-8a33-8dff90194f74" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_bae56a3e-b451-4e80-8a33-8dff90194f74_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_5fe8b2da-82a6-47be-a682-1b68b1703051" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_bae56a3e-b451-4e80-8a33-8dff90194f74" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_5fe8b2da-82a6-47be-a682-1b68b1703051" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_086d8719-aa22-4f07-94a6-44837a45ec31" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_5fe8b2da-82a6-47be-a682-1b68b1703051" xlink:to="loc_us-gaap_CommercialPaperMember_086d8719-aa22-4f07-94a6-44837a45ec31" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_de01c547-eed4-4c21-bd61-73c49e15f248" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_5fe8b2da-82a6-47be-a682-1b68b1703051" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_de01c547-eed4-4c21-bd61-73c49e15f248" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasurySecuritiesMember_21d1ee29-1f63-4262-a761-3bead63fd6d4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_USTreasurySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_5fe8b2da-82a6-47be-a682-1b68b1703051" xlink:to="loc_us-gaap_USTreasurySecuritiesMember_21d1ee29-1f63-4262-a761-3bead63fd6d4" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://arcutis.com/role/FairValueMeasurementsEstimatedValueofCashandCashEquivalentsandMarketableSecuritiesDetails" xlink:type="simple" xlink:href="arqt-20210930.xsd#FairValueMeasurementsEstimatedValueofCashandCashEquivalentsandMarketableSecuritiesDetails"/>
  <link:definitionLink xlink:role="http://arcutis.com/role/FairValueMeasurementsEstimatedValueofCashandCashEquivalentsandMarketableSecuritiesDetails" xlink:type="extended" id="i657ad41150234589b06749a70c1365ce_FairValueMeasurementsEstimatedValueofCashandCashEquivalentsandMarketableSecuritiesDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsLineItems_60f2e46d-37db-4a5a-8a47-bcb63a0ac5df" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CashAndCashEquivalentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_beb119d9-fbb2-4494-b09c-fb4b3533ee5b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_60f2e46d-37db-4a5a-8a47-bcb63a0ac5df" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_beb119d9-fbb2-4494-b09c-fb4b3533ee5b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_d5834e9a-27e8-4953-a947-c9eb39ce51c2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_60f2e46d-37db-4a5a-8a47-bcb63a0ac5df" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_d5834e9a-27e8-4953-a947-c9eb39ce51c2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_d6f98718-7f89-4387-92c5-1e4bcec302db" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfCashAndCashEquivalentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_60f2e46d-37db-4a5a-8a47-bcb63a0ac5df" xlink:to="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_d6f98718-7f89-4387-92c5-1e4bcec302db" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAxis_3e92f7d4-3351-4878-aa0a-25f7c979c1ce" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CashAndCashEquivalentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_d6f98718-7f89-4387-92c5-1e4bcec302db" xlink:to="loc_us-gaap_CashAndCashEquivalentsAxis_3e92f7d4-3351-4878-aa0a-25f7c979c1ce" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_3e92f7d4-3351-4878-aa0a-25f7c979c1ce_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_3e92f7d4-3351-4878-aa0a-25f7c979c1ce" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_3e92f7d4-3351-4878-aa0a-25f7c979c1ce_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_e8a15e64-5bd3-40b3-a730-84579a011978" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_3e92f7d4-3351-4878-aa0a-25f7c979c1ce" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_e8a15e64-5bd3-40b3-a730-84579a011978" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsMember_8bb82143-9221-4ed6-a519-989daa041408" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_MoneyMarketFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_e8a15e64-5bd3-40b3-a730-84579a011978" xlink:to="loc_us-gaap_MoneyMarketFundsMember_8bb82143-9221-4ed6-a519-989daa041408" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_0a869290-702a-42d7-b945-fd01150f7637" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_22ba8dc6-7404-4bb0-a84e-4a6ba422e7a0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_0a869290-702a-42d7-b945-fd01150f7637" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_22ba8dc6-7404-4bb0-a84e-4a6ba422e7a0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_2a67db42-b667-461a-92fa-1f35552f2ff0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_0a869290-702a-42d7-b945-fd01150f7637" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_2a67db42-b667-461a-92fa-1f35552f2ff0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_a653ddd1-0b08-4285-b91c-52f0c35bc77e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_0a869290-702a-42d7-b945-fd01150f7637" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_a653ddd1-0b08-4285-b91c-52f0c35bc77e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_a3759fa0-52a5-49c4-8c4e-d9d786a36f6b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_0a869290-702a-42d7-b945-fd01150f7637" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_a3759fa0-52a5-49c4-8c4e-d9d786a36f6b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_ebf82e47-5ea6-466a-a792-756d1a8c05e5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtSecuritiesAvailableForSaleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_0a869290-702a-42d7-b945-fd01150f7637" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_ebf82e47-5ea6-466a-a792-756d1a8c05e5" xlink:type="arc" order="2" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_2edce5eb-d6e8-4550-b315-2ea46cd837f9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_ebf82e47-5ea6-466a-a792-756d1a8c05e5" xlink:to="loc_us-gaap_FinancialInstrumentAxis_2edce5eb-d6e8-4550-b315-2ea46cd837f9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_2edce5eb-d6e8-4550-b315-2ea46cd837f9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_2edce5eb-d6e8-4550-b315-2ea46cd837f9" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_2edce5eb-d6e8-4550-b315-2ea46cd837f9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_bdf7fdf0-f57e-45d2-a077-70dce5d46ceb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_2edce5eb-d6e8-4550-b315-2ea46cd837f9" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_bdf7fdf0-f57e-45d2-a077-70dce5d46ceb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_04269bce-e9f7-4395-9e1c-ae4dccd8369c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_bdf7fdf0-f57e-45d2-a077-70dce5d46ceb" xlink:to="loc_us-gaap_CommercialPaperMember_04269bce-e9f7-4395-9e1c-ae4dccd8369c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_77ffe539-8639-4c32-b537-a0530673ef1d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_bdf7fdf0-f57e-45d2-a077-70dce5d46ceb" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_77ffe539-8639-4c32-b537-a0530673ef1d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasurySecuritiesMember_630d6461-f92d-4b7b-ab3d-ac5799302d98" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_USTreasurySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_bdf7fdf0-f57e-45d2-a077-70dce5d46ceb" xlink:to="loc_us-gaap_USTreasurySecuritiesMember_630d6461-f92d-4b7b-ab3d-ac5799302d98" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://arcutis.com/role/PropertyandEquipmentnetSummaryDetails" xlink:type="simple" xlink:href="arqt-20210930.xsd#PropertyandEquipmentnetSummaryDetails"/>
  <link:definitionLink xlink:role="http://arcutis.com/role/PropertyandEquipmentnetSummaryDetails" xlink:type="extended" id="idc91955e722b414bb9b3ec33426e28ed_PropertyandEquipmentnetSummaryDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_c8ec4595-879b-42ef-b676-f13f607c0098" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_b00a6d50-d2f6-4b53-8d28-25912cc8f42d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_c8ec4595-879b-42ef-b676-f13f607c0098" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_b00a6d50-d2f6-4b53-8d28-25912cc8f42d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_2d88411f-51ea-4eaf-8f1e-df1aff842c93" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_c8ec4595-879b-42ef-b676-f13f607c0098" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_2d88411f-51ea-4eaf-8f1e-df1aff842c93" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_7ce207d3-dda5-48bb-a223-35e78cbfd036" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_c8ec4595-879b-42ef-b676-f13f607c0098" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_7ce207d3-dda5-48bb-a223-35e78cbfd036" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_617ae2c0-9b99-4bba-a802-894e5cff28d6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_c8ec4595-879b-42ef-b676-f13f607c0098" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_617ae2c0-9b99-4bba-a802-894e5cff28d6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_a4bb89ec-5b7f-4afa-8aef-832508378bc6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_c8ec4595-879b-42ef-b676-f13f607c0098" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_a4bb89ec-5b7f-4afa-8aef-832508378bc6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_eaecfd6d-5bd6-45cb-a30b-fc29df7e0ed9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_a4bb89ec-5b7f-4afa-8aef-832508378bc6" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_eaecfd6d-5bd6-45cb-a30b-fc29df7e0ed9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_eaecfd6d-5bd6-45cb-a30b-fc29df7e0ed9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_eaecfd6d-5bd6-45cb-a30b-fc29df7e0ed9" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_eaecfd6d-5bd6-45cb-a30b-fc29df7e0ed9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_8add1656-c9b9-47f2-bc8d-cd45af1809ca" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_eaecfd6d-5bd6-45cb-a30b-fc29df7e0ed9" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_8add1656-c9b9-47f2-bc8d-cd45af1809ca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComputerEquipmentMember_3580d5b8-24ae-4499-837c-ee5f54f8cdeb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ComputerEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_8add1656-c9b9-47f2-bc8d-cd45af1809ca" xlink:to="loc_us-gaap_ComputerEquipmentMember_3580d5b8-24ae-4499-837c-ee5f54f8cdeb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_f8b44755-c2aa-4be8-805f-c3744d02642e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FurnitureAndFixturesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_8add1656-c9b9-47f2-bc8d-cd45af1809ca" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_f8b44755-c2aa-4be8-805f-c3744d02642e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SoftwareAndSoftwareDevelopmentCostsMember_38ae5cf5-9505-4349-ac2a-3b58d68f8466" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SoftwareAndSoftwareDevelopmentCostsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_8add1656-c9b9-47f2-bc8d-cd45af1809ca" xlink:to="loc_us-gaap_SoftwareAndSoftwareDevelopmentCostsMember_38ae5cf5-9505-4349-ac2a-3b58d68f8466" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConstructionInProgressMember_b4b6c584-a53c-4218-afd8-7a58466ef29b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ConstructionInProgressMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_8add1656-c9b9-47f2-bc8d-cd45af1809ca" xlink:to="loc_us-gaap_ConstructionInProgressMember_b4b6c584-a53c-4218-afd8-7a58466ef29b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseholdImprovementsMember_bdd5c109-6332-4d8e-bf56-426ec295cbeb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LeaseholdImprovementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_8add1656-c9b9-47f2-bc8d-cd45af1809ca" xlink:to="loc_us-gaap_LeaseholdImprovementsMember_bdd5c109-6332-4d8e-bf56-426ec295cbeb" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://arcutis.com/role/LicenseAgreementsAstraZenecaDetails" xlink:type="simple" xlink:href="arqt-20210930.xsd#LicenseAgreementsAstraZenecaDetails"/>
  <link:definitionLink xlink:role="http://arcutis.com/role/LicenseAgreementsAstraZenecaDetails" xlink:type="extended" id="ie96941d0c5204537b3a5a3e1cc4f05ec_LicenseAgreementsAstraZenecaDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_3e62ec13-eb3f-4dce-92cc-e999b8e9ed03" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersCostsIncurredGross_ce696db7-5e73-4247-b098-1068907ce619" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersCostsIncurredGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_3e62ec13-eb3f-4dce-92cc-e999b8e9ed03" xlink:to="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersCostsIncurredGross_ce696db7-5e73-4247-b098-1068907ce619" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_TemporaryEquityStockIssuedDuringPeriodSharesNewIssues_7f38bae4-c14f-4149-a735-b7822b2d8205" xlink:href="arqt-20210930.xsd#arqt_TemporaryEquityStockIssuedDuringPeriodSharesNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_3e62ec13-eb3f-4dce-92cc-e999b8e9ed03" xlink:to="loc_arqt_TemporaryEquityStockIssuedDuringPeriodSharesNewIssues_7f38bae4-c14f-4149-a735-b7822b2d8205" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityStockIssuedDuringPeriodValueNewIssues_4003c233-d621-49b2-868d-7b47baae6258" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TemporaryEquityStockIssuedDuringPeriodValueNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_3e62ec13-eb3f-4dce-92cc-e999b8e9ed03" xlink:to="loc_us-gaap_TemporaryEquityStockIssuedDuringPeriodValueNewIssues_4003c233-d621-49b2-868d-7b47baae6258" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_ResearchAndDevelopmentArrangementContractToPerformForOthersMaximumProductMilestonePayments_f073244a-149c-4318-985d-5aa978da1992" xlink:href="arqt-20210930.xsd#arqt_ResearchAndDevelopmentArrangementContractToPerformForOthersMaximumProductMilestonePayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_3e62ec13-eb3f-4dce-92cc-e999b8e9ed03" xlink:to="loc_arqt_ResearchAndDevelopmentArrangementContractToPerformForOthersMaximumProductMilestonePayments_f073244a-149c-4318-985d-5aa978da1992" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_ResearchAndDevelopmentArrangementContractToPerformForOthersMaximumProductMilestonePaymentsUponFirstProductApproval_80dd9620-32cc-49cd-ace1-07e16ff9c4f1" xlink:href="arqt-20210930.xsd#arqt_ResearchAndDevelopmentArrangementContractToPerformForOthersMaximumProductMilestonePaymentsUponFirstProductApproval"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_3e62ec13-eb3f-4dce-92cc-e999b8e9ed03" xlink:to="loc_arqt_ResearchAndDevelopmentArrangementContractToPerformForOthersMaximumProductMilestonePaymentsUponFirstProductApproval_80dd9620-32cc-49cd-ace1-07e16ff9c4f1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_ResearchAndDevelopmentArrangementContractToPerformForOthersMaximumSalesMilestonePayments_4d93b20d-d5f2-4db6-a000-663cbd919359" xlink:href="arqt-20210930.xsd#arqt_ResearchAndDevelopmentArrangementContractToPerformForOthersMaximumSalesMilestonePayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_3e62ec13-eb3f-4dce-92cc-e999b8e9ed03" xlink:to="loc_arqt_ResearchAndDevelopmentArrangementContractToPerformForOthersMaximumSalesMilestonePayments_4d93b20d-d5f2-4db6-a000-663cbd919359" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfResearchAndDevelopmentArrangementContractToPerformForOthersTable_dbff1ff7-ac2a-4006-8ce5-3428cd164c0a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfResearchAndDevelopmentArrangementContractToPerformForOthersTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_3e62ec13-eb3f-4dce-92cc-e999b8e9ed03" xlink:to="loc_us-gaap_ScheduleOfResearchAndDevelopmentArrangementContractToPerformForOthersTable_dbff1ff7-ac2a-4006-8ce5-3428cd164c0a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_0251ee3f-7e03-4e95-bd0b-74a6602c0223" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfResearchAndDevelopmentArrangementContractToPerformForOthersTable_dbff1ff7-ac2a-4006-8ce5-3428cd164c0a" xlink:to="loc_us-gaap_StatementClassOfStockAxis_0251ee3f-7e03-4e95-bd0b-74a6602c0223" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_0251ee3f-7e03-4e95-bd0b-74a6602c0223_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_0251ee3f-7e03-4e95-bd0b-74a6602c0223" xlink:to="loc_us-gaap_ClassOfStockDomain_0251ee3f-7e03-4e95-bd0b-74a6602c0223_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_59aa5baf-d4bb-4f60-b595-202fc3a5b67b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_0251ee3f-7e03-4e95-bd0b-74a6602c0223" xlink:to="loc_us-gaap_ClassOfStockDomain_59aa5baf-d4bb-4f60-b595-202fc3a5b67b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeriesBPreferredStockMember_20585cdb-b170-4f98-896b-5de4bdb10c78" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SeriesBPreferredStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_59aa5baf-d4bb-4f60-b595-202fc3a5b67b" xlink:to="loc_us-gaap_SeriesBPreferredStockMember_20585cdb-b170-4f98-896b-5de4bdb10c78" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_8c22e902-f6b4-4350-b2ee-c08655fe7f10" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfResearchAndDevelopmentArrangementContractToPerformForOthersTable_dbff1ff7-ac2a-4006-8ce5-3428cd164c0a" xlink:to="loc_srt_CounterpartyNameAxis_8c22e902-f6b4-4350-b2ee-c08655fe7f10" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_8c22e902-f6b4-4350-b2ee-c08655fe7f10_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_8c22e902-f6b4-4350-b2ee-c08655fe7f10" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_8c22e902-f6b4-4350-b2ee-c08655fe7f10_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_b6fd2b4a-3914-45b2-b0b8-d76043c32a58" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_8c22e902-f6b4-4350-b2ee-c08655fe7f10" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_b6fd2b4a-3914-45b2-b0b8-d76043c32a58" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_AstraZenecaMember_6332f28c-d302-4f9a-887f-baa2b99f0ad5" xlink:href="arqt-20210930.xsd#arqt_AstraZenecaMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_b6fd2b4a-3914-45b2-b0b8-d76043c32a58" xlink:to="loc_arqt_AstraZenecaMember_6332f28c-d302-4f9a-887f-baa2b99f0ad5" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://arcutis.com/role/LicenseAgreementsHengruiDetails" xlink:type="simple" xlink:href="arqt-20210930.xsd#LicenseAgreementsHengruiDetails"/>
  <link:definitionLink xlink:role="http://arcutis.com/role/LicenseAgreementsHengruiDetails" xlink:type="extended" id="i9a002d814ac84e1791bda2b5080998b4_LicenseAgreementsHengruiDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_ec1f39f3-5016-4239-8634-ac04bd76ad24" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersCostsIncurredGross_0d1cf203-a904-4906-a161-f72fe21437da" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersCostsIncurredGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_ec1f39f3-5016-4239-8634-ac04bd76ad24" xlink:to="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersCostsIncurredGross_0d1cf203-a904-4906-a161-f72fe21437da" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_ResearchAndDevelopmentArrangementContractToPerformForOthersMaximumProductMilestonePayments_f9475d5d-3504-4197-b2b3-2663e318812c" xlink:href="arqt-20210930.xsd#arqt_ResearchAndDevelopmentArrangementContractToPerformForOthersMaximumProductMilestonePayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_ec1f39f3-5016-4239-8634-ac04bd76ad24" xlink:to="loc_arqt_ResearchAndDevelopmentArrangementContractToPerformForOthersMaximumProductMilestonePayments_f9475d5d-3504-4197-b2b3-2663e318812c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_ResearchAndDevelopmentArrangementContractToPerformForOthersMaximumSalesMilestonePayments_00cb4c62-9623-49d6-93ba-6c655dd4f647" xlink:href="arqt-20210930.xsd#arqt_ResearchAndDevelopmentArrangementContractToPerformForOthersMaximumSalesMilestonePayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_ec1f39f3-5016-4239-8634-ac04bd76ad24" xlink:to="loc_arqt_ResearchAndDevelopmentArrangementContractToPerformForOthersMaximumSalesMilestonePayments_00cb4c62-9623-49d6-93ba-6c655dd4f647" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfResearchAndDevelopmentArrangementContractToPerformForOthersTable_a1417c30-dd70-4003-b6d3-10e9ce2508a5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfResearchAndDevelopmentArrangementContractToPerformForOthersTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_ec1f39f3-5016-4239-8634-ac04bd76ad24" xlink:to="loc_us-gaap_ScheduleOfResearchAndDevelopmentArrangementContractToPerformForOthersTable_a1417c30-dd70-4003-b6d3-10e9ce2508a5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_8f3e717e-d23a-48db-abd4-ac9965b92b1f" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfResearchAndDevelopmentArrangementContractToPerformForOthersTable_a1417c30-dd70-4003-b6d3-10e9ce2508a5" xlink:to="loc_srt_CounterpartyNameAxis_8f3e717e-d23a-48db-abd4-ac9965b92b1f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_8f3e717e-d23a-48db-abd4-ac9965b92b1f_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_8f3e717e-d23a-48db-abd4-ac9965b92b1f" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_8f3e717e-d23a-48db-abd4-ac9965b92b1f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_3423544d-470a-41a9-98bf-b6e0de671a2b" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_8f3e717e-d23a-48db-abd4-ac9965b92b1f" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_3423544d-470a-41a9-98bf-b6e0de671a2b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_HengruiMember_ffb17a2c-366b-40b5-a7e0-debf7862495a" xlink:href="arqt-20210930.xsd#arqt_HengruiMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_3423544d-470a-41a9-98bf-b6e0de671a2b" xlink:to="loc_arqt_HengruiMember_ffb17a2c-366b-40b5-a7e0-debf7862495a" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://arcutis.com/role/LicenseAgreementsHawkeyeDetails" xlink:type="simple" xlink:href="arqt-20210930.xsd#LicenseAgreementsHawkeyeDetails"/>
  <link:definitionLink xlink:role="http://arcutis.com/role/LicenseAgreementsHawkeyeDetails" xlink:type="extended" id="i512b576afb9746a583ffff85d73f4140_LicenseAgreementsHawkeyeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableInterestEntityLineItems_876ae640-94af-4000-a1cb-295938a36fac" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_VariableInterestEntityLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_VariableInterestEntitiesNumberOfSharesHeld_89289256-a6be-434e-ba54-2696aedd38e7" xlink:href="arqt-20210930.xsd#arqt_VariableInterestEntitiesNumberOfSharesHeld"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_876ae640-94af-4000-a1cb-295938a36fac" xlink:to="loc_arqt_VariableInterestEntitiesNumberOfSharesHeld_89289256-a6be-434e-ba54-2696aedd38e7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_c4760149-2b93-4225-9ce2-86e6302699d9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_876ae640-94af-4000-a1cb-295938a36fac" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_c4760149-2b93-4225-9ce2-86e6302699d9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableInterestEntityOwnershipPercentage_c7d8e635-b3b9-49da-ae25-1ee279bdc695" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_VariableInterestEntityOwnershipPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_876ae640-94af-4000-a1cb-295938a36fac" xlink:to="loc_us-gaap_VariableInterestEntityOwnershipPercentage_c7d8e635-b3b9-49da-ae25-1ee279bdc695" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_CollaborationAgreementThresholdOfProceedsFromStockIssuanceForAdditionalConsideration_455d84bb-1d3b-47bc-8c2e-1dcf11cc4ff2" xlink:href="arqt-20210930.xsd#arqt_CollaborationAgreementThresholdOfProceedsFromStockIssuanceForAdditionalConsideration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_876ae640-94af-4000-a1cb-295938a36fac" xlink:to="loc_arqt_CollaborationAgreementThresholdOfProceedsFromStockIssuanceForAdditionalConsideration_455d84bb-1d3b-47bc-8c2e-1dcf11cc4ff2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_CollaborativeArrangementNumeratorForDeterminingAdditionalConsideration_edaf262a-12d9-4053-82c5-1f21408782a5" xlink:href="arqt-20210930.xsd#arqt_CollaborativeArrangementNumeratorForDeterminingAdditionalConsideration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_876ae640-94af-4000-a1cb-295938a36fac" xlink:to="loc_arqt_CollaborativeArrangementNumeratorForDeterminingAdditionalConsideration_edaf262a-12d9-4053-82c5-1f21408782a5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_ResearchAndDevelopmentArrangementPaymentsMilestonesOrRoyalties_205f5fb3-f0ab-4d7f-94dd-0192ff49ba78" xlink:href="arqt-20210930.xsd#arqt_ResearchAndDevelopmentArrangementPaymentsMilestonesOrRoyalties"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_876ae640-94af-4000-a1cb-295938a36fac" xlink:to="loc_arqt_ResearchAndDevelopmentArrangementPaymentsMilestonesOrRoyalties_205f5fb3-f0ab-4d7f-94dd-0192ff49ba78" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_123ef842-15cb-450e-bb85-e8caa02b7ce0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfVariableInterestEntitiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_VariableInterestEntityLineItems_876ae640-94af-4000-a1cb-295938a36fac" xlink:to="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_123ef842-15cb-450e-bb85-e8caa02b7ce0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_7ce26c78-f4a6-405f-95eb-5e52c5f34c49" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_123ef842-15cb-450e-bb85-e8caa02b7ce0" xlink:to="loc_dei_LegalEntityAxis_7ce26c78-f4a6-405f-95eb-5e52c5f34c49" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_7ce26c78-f4a6-405f-95eb-5e52c5f34c49_default" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_7ce26c78-f4a6-405f-95eb-5e52c5f34c49" xlink:to="loc_dei_EntityDomain_7ce26c78-f4a6-405f-95eb-5e52c5f34c49_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_c1addbf9-c36c-4c89-a2fe-fe877727c544" xlink:href="https://xbrl.sec.gov/dei/2021/dei-2021.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_7ce26c78-f4a6-405f-95eb-5e52c5f34c49" xlink:to="loc_dei_EntityDomain_c1addbf9-c36c-4c89-a2fe-fe877727c544" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_HawkeyeMember_d1d3c8cd-2702-4b43-b44b-63e4e026171e" xlink:href="arqt-20210930.xsd#arqt_HawkeyeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_c1addbf9-c36c-4c89-a2fe-fe877727c544" xlink:to="loc_arqt_HawkeyeMember_d1d3c8cd-2702-4b43-b44b-63e4e026171e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_f58f7913-f83b-462c-9d62-566215b962c1" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfVariableInterestEntitiesTable_123ef842-15cb-450e-bb85-e8caa02b7ce0" xlink:to="loc_srt_CounterpartyNameAxis_f58f7913-f83b-462c-9d62-566215b962c1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_f58f7913-f83b-462c-9d62-566215b962c1_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_f58f7913-f83b-462c-9d62-566215b962c1" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_f58f7913-f83b-462c-9d62-566215b962c1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_7bee581e-2430-488f-b3f3-74afe587106a" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_f58f7913-f83b-462c-9d62-566215b962c1" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_7bee581e-2430-488f-b3f3-74afe587106a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_HawkeyeMember_96ca624b-a57e-4838-88c4-4fd970667767" xlink:href="arqt-20210930.xsd#arqt_HawkeyeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_7bee581e-2430-488f-b3f3-74afe587106a" xlink:to="loc_arqt_HawkeyeMember_96ca624b-a57e-4838-88c4-4fd970667767" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://arcutis.com/role/CommitmentsandContingenciesNarrativeDetails" xlink:type="simple" xlink:href="arqt-20210930.xsd#CommitmentsandContingenciesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://arcutis.com/role/CommitmentsandContingenciesNarrativeDetails" xlink:type="extended" id="ia96144a931734cc3ac6df980c6b4e0bd_CommitmentsandContingenciesNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionLineItems_06c40bd5-ba60-4674-b45c-085f3e8f8bc7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LesseeLeaseDescriptionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_LesseeOperatingLeaseSquareFootageOfLeasedSpace_4e6c89c0-e12f-4fba-bc6e-c2c1fd82973c" xlink:href="arqt-20210930.xsd#arqt_LesseeOperatingLeaseSquareFootageOfLeasedSpace"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_06c40bd5-ba60-4674-b45c-085f3e8f8bc7" xlink:to="loc_arqt_LesseeOperatingLeaseSquareFootageOfLeasedSpace_4e6c89c0-e12f-4fba-bc6e-c2c1fd82973c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_ReductionOfRightOfUseAssetUponReassessmentOfLeaseTerm_8a08b61c-653f-437b-96e1-00533bd284f5" xlink:href="arqt-20210930.xsd#arqt_ReductionOfRightOfUseAssetUponReassessmentOfLeaseTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_06c40bd5-ba60-4674-b45c-085f3e8f8bc7" xlink:to="loc_arqt_ReductionOfRightOfUseAssetUponReassessmentOfLeaseTerm_8a08b61c-653f-437b-96e1-00533bd284f5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_LesseeOperatingLeasePaymentPeriodAfterTenantImprovements_ab7a5b82-fe72-4ca1-bb09-d025b3b27b6d" xlink:href="arqt-20210930.xsd#arqt_LesseeOperatingLeasePaymentPeriodAfterTenantImprovements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_06c40bd5-ba60-4674-b45c-085f3e8f8bc7" xlink:to="loc_arqt_LesseeOperatingLeasePaymentPeriodAfterTenantImprovements_ab7a5b82-fe72-4ca1-bb09-d025b3b27b6d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseTermOfContract_aeafb71a-e514-476b-a6b4-aa4e2d575bff" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LesseeOperatingLeaseTermOfContract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_06c40bd5-ba60-4674-b45c-085f3e8f8bc7" xlink:to="loc_us-gaap_LesseeOperatingLeaseTermOfContract_aeafb71a-e514-476b-a6b4-aa4e2d575bff" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseRenewalTerm_c5431707-75df-4b12-b19d-d3d96161d24b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LesseeOperatingLeaseRenewalTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_06c40bd5-ba60-4674-b45c-085f3e8f8bc7" xlink:to="loc_us-gaap_LesseeOperatingLeaseRenewalTerm_c5431707-75df-4b12-b19d-d3d96161d24b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_LesseeOperatingLeaseTermForTerminationOption_f91c3bf1-cce7-48b0-aa9b-d11bd026d74a" xlink:href="arqt-20210930.xsd#arqt_LesseeOperatingLeaseTermForTerminationOption"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_06c40bd5-ba60-4674-b45c-085f3e8f8bc7" xlink:to="loc_arqt_LesseeOperatingLeaseTermForTerminationOption_f91c3bf1-cce7-48b0-aa9b-d11bd026d74a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_LesseeOperatingLeaseInitialBaseRent_69c5ab06-9f0a-4779-996b-8b0bd72cf5b7" xlink:href="arqt-20210930.xsd#arqt_LesseeOperatingLeaseInitialBaseRent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_06c40bd5-ba60-4674-b45c-085f3e8f8bc7" xlink:to="loc_arqt_LesseeOperatingLeaseInitialBaseRent_69c5ab06-9f0a-4779-996b-8b0bd72cf5b7" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_LesseeOperatingLeaseFreeRentPeriod_b1ec0dd1-6f00-45ce-a9ca-b73f5b54be97" xlink:href="arqt-20210930.xsd#arqt_LesseeOperatingLeaseFreeRentPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_06c40bd5-ba60-4674-b45c-085f3e8f8bc7" xlink:to="loc_arqt_LesseeOperatingLeaseFreeRentPeriod_b1ec0dd1-6f00-45ce-a9ca-b73f5b54be97" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_LesseeOperatingLeaseTenantImprovementAllowance_a4fe27cc-5e69-4ad6-a0ad-60ae16851c0c" xlink:href="arqt-20210930.xsd#arqt_LesseeOperatingLeaseTenantImprovementAllowance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_06c40bd5-ba60-4674-b45c-085f3e8f8bc7" xlink:to="loc_arqt_LesseeOperatingLeaseTenantImprovementAllowance_a4fe27cc-5e69-4ad6-a0ad-60ae16851c0c" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_LesseeOperatingLeaseLetterOfCreditRequirement_dca37cbe-330c-4d87-a208-308f5ec2335a" xlink:href="arqt-20210930.xsd#arqt_LesseeOperatingLeaseLetterOfCreditRequirement"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_06c40bd5-ba60-4674-b45c-085f3e8f8bc7" xlink:to="loc_arqt_LesseeOperatingLeaseLetterOfCreditRequirement_dca37cbe-330c-4d87-a208-308f5ec2335a" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalents_975c3fd0-b1c1-4d8a-96a9-5cb6fa22e9c4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_06c40bd5-ba60-4674-b45c-085f3e8f8bc7" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalents_975c3fd0-b1c1-4d8a-96a9-5cb6fa22e9c4" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_IncreaseDecreaseInLetterOfCreditAndRestrictedCash_1e325a9d-e880-4caa-91bc-a3246415f68a" xlink:href="arqt-20210930.xsd#arqt_IncreaseDecreaseInLetterOfCreditAndRestrictedCash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_06c40bd5-ba60-4674-b45c-085f3e8f8bc7" xlink:to="loc_arqt_IncreaseDecreaseInLetterOfCreditAndRestrictedCash_1e325a9d-e880-4caa-91bc-a3246415f68a" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseOfRestrictedInvestments_952813de-c366-49e9-b8a2-0f92aab4d069" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncreaseDecreaseOfRestrictedInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_06c40bd5-ba60-4674-b45c-085f3e8f8bc7" xlink:to="loc_us-gaap_IncreaseDecreaseOfRestrictedInvestments_952813de-c366-49e9-b8a2-0f92aab4d069" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiability_49563003-7536-476b-9e7a-934741bbdfea" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_06c40bd5-ba60-4674-b45c-085f3e8f8bc7" xlink:to="loc_us-gaap_OperatingLeaseLiability_49563003-7536-476b-9e7a-934741bbdfea" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAsset_6a2557d4-d029-4cfd-9134-bf433b47fc15" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_06c40bd5-ba60-4674-b45c-085f3e8f8bc7" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAsset_6a2557d4-d029-4cfd-9134-bf433b47fc15" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_LesseeOperatingLeaseTenantImprovementAllowanceCapitalizationInExcess_946b6aa4-1942-4f3a-a9cb-0dea87d2804d" xlink:href="arqt-20210930.xsd#arqt_LesseeOperatingLeaseTenantImprovementAllowanceCapitalizationInExcess"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_06c40bd5-ba60-4674-b45c-085f3e8f8bc7" xlink:to="loc_arqt_LesseeOperatingLeaseTenantImprovementAllowanceCapitalizationInExcess_946b6aa4-1942-4f3a-a9cb-0dea87d2804d" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseExpense_b2833e21-9aea-41e4-ba83-ad96c325255a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingLeaseExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_06c40bd5-ba60-4674-b45c-085f3e8f8bc7" xlink:to="loc_us-gaap_OperatingLeaseExpense_b2833e21-9aea-41e4-ba83-ad96c325255a" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableLeaseCost_fbe69d2f-8009-44cd-bc79-9bd3e7ee095a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_VariableLeaseCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_06c40bd5-ba60-4674-b45c-085f3e8f8bc7" xlink:to="loc_us-gaap_VariableLeaseCost_fbe69d2f-8009-44cd-bc79-9bd3e7ee095a" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PurchaseObligationFutureMinimumPaymentsRemainderOfFiscalYear_6c66959f-5ac2-44a9-9629-1a384ce80c50" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PurchaseObligationFutureMinimumPaymentsRemainderOfFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_06c40bd5-ba60-4674-b45c-085f3e8f8bc7" xlink:to="loc_us-gaap_PurchaseObligationFutureMinimumPaymentsRemainderOfFiscalYear_6c66959f-5ac2-44a9-9629-1a384ce80c50" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PurchaseObligationDueInNextTwelveMonths_7cd62b09-a353-4f41-abe0-cda1112280fe" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PurchaseObligationDueInNextTwelveMonths"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_06c40bd5-ba60-4674-b45c-085f3e8f8bc7" xlink:to="loc_us-gaap_PurchaseObligationDueInNextTwelveMonths_7cd62b09-a353-4f41-abe0-cda1112280fe" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PurchaseObligationDueInSecondYear_ceaecc9a-1ba4-447c-a4c6-65d095c32823" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PurchaseObligationDueInSecondYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_06c40bd5-ba60-4674-b45c-085f3e8f8bc7" xlink:to="loc_us-gaap_PurchaseObligationDueInSecondYear_ceaecc9a-1ba4-447c-a4c6-65d095c32823" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PurchaseObligationDueInThirdYear_e056143c-9807-4da4-aee9-63e0b9c06b16" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PurchaseObligationDueInThirdYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_06c40bd5-ba60-4674-b45c-085f3e8f8bc7" xlink:to="loc_us-gaap_PurchaseObligationDueInThirdYear_e056143c-9807-4da4-aee9-63e0b9c06b16" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PurchaseObligationDueInFourthYear_74931849-6c47-482a-ae55-48f24ee66e38" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PurchaseObligationDueInFourthYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_06c40bd5-ba60-4674-b45c-085f3e8f8bc7" xlink:to="loc_us-gaap_PurchaseObligationDueInFourthYear_74931849-6c47-482a-ae55-48f24ee66e38" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionTable_759386fd-2634-4c91-8386-4ffb5f4fa669" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LesseeLeaseDescriptionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_06c40bd5-ba60-4674-b45c-085f3e8f8bc7" xlink:to="loc_us-gaap_LesseeLeaseDescriptionTable_759386fd-2634-4c91-8386-4ffb5f4fa669" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_LeaseDescriptionAxis_159a98a7-af4f-4d54-9b69-5cc32d29e6f8" xlink:href="arqt-20210930.xsd#arqt_LeaseDescriptionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_759386fd-2634-4c91-8386-4ffb5f4fa669" xlink:to="loc_arqt_LeaseDescriptionAxis_159a98a7-af4f-4d54-9b69-5cc32d29e6f8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_LeaseDescriptionDomain_159a98a7-af4f-4d54-9b69-5cc32d29e6f8_default" xlink:href="arqt-20210930.xsd#arqt_LeaseDescriptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_arqt_LeaseDescriptionAxis_159a98a7-af4f-4d54-9b69-5cc32d29e6f8" xlink:to="loc_arqt_LeaseDescriptionDomain_159a98a7-af4f-4d54-9b69-5cc32d29e6f8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_LeaseDescriptionDomain_7cb3c9f8-765d-4400-9a8b-4382fe630909" xlink:href="arqt-20210930.xsd#arqt_LeaseDescriptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_arqt_LeaseDescriptionAxis_159a98a7-af4f-4d54-9b69-5cc32d29e6f8" xlink:to="loc_arqt_LeaseDescriptionDomain_7cb3c9f8-765d-4400-9a8b-4382fe630909" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_WestlakeVillageCaliforniaLeaseArrangementMember_40e2317d-df0f-4812-a571-2e8ac589a937" xlink:href="arqt-20210930.xsd#arqt_WestlakeVillageCaliforniaLeaseArrangementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_arqt_LeaseDescriptionDomain_7cb3c9f8-765d-4400-9a8b-4382fe630909" xlink:to="loc_arqt_WestlakeVillageCaliforniaLeaseArrangementMember_40e2317d-df0f-4812-a571-2e8ac589a937" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseContractualTermAxis_ad6a0350-2c58-4870-ac5d-d0accf63952a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LeaseContractualTermAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_759386fd-2634-4c91-8386-4ffb5f4fa669" xlink:to="loc_us-gaap_LeaseContractualTermAxis_ad6a0350-2c58-4870-ac5d-d0accf63952a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseContractualTermDomain_ad6a0350-2c58-4870-ac5d-d0accf63952a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LeaseContractualTermDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LeaseContractualTermAxis_ad6a0350-2c58-4870-ac5d-d0accf63952a" xlink:to="loc_us-gaap_LeaseContractualTermDomain_ad6a0350-2c58-4870-ac5d-d0accf63952a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseContractualTermDomain_24e2e76f-4c1f-47b0-8a76-5ea291c6ee77" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LeaseContractualTermDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LeaseContractualTermAxis_ad6a0350-2c58-4870-ac5d-d0accf63952a" xlink:to="loc_us-gaap_LeaseContractualTermDomain_24e2e76f-4c1f-47b0-8a76-5ea291c6ee77" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_FirstAnniversaryMember_8af56674-2e99-4970-9085-0a7b8526cec3" xlink:href="arqt-20210930.xsd#arqt_FirstAnniversaryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeaseContractualTermDomain_24e2e76f-4c1f-47b0-8a76-5ea291c6ee77" xlink:to="loc_arqt_FirstAnniversaryMember_8af56674-2e99-4970-9085-0a7b8526cec3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_SecondAnniversaryMember_2dda99fe-f6a5-493a-970d-8c65bbfe6378" xlink:href="arqt-20210930.xsd#arqt_SecondAnniversaryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeaseContractualTermDomain_24e2e76f-4c1f-47b0-8a76-5ea291c6ee77" xlink:to="loc_arqt_SecondAnniversaryMember_2dda99fe-f6a5-493a-970d-8c65bbfe6378" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_ThirdAnniversaryMember_c8410ed0-15b6-4e3f-8b09-4d48e651daa9" xlink:href="arqt-20210930.xsd#arqt_ThirdAnniversaryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeaseContractualTermDomain_24e2e76f-4c1f-47b0-8a76-5ea291c6ee77" xlink:to="loc_arqt_ThirdAnniversaryMember_c8410ed0-15b6-4e3f-8b09-4d48e651daa9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_FourthAnniversaryMember_9cd1663b-0d9e-4a02-bdc0-bed7f1719d3b" xlink:href="arqt-20210930.xsd#arqt_FourthAnniversaryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeaseContractualTermDomain_24e2e76f-4c1f-47b0-8a76-5ea291c6ee77" xlink:to="loc_arqt_FourthAnniversaryMember_9cd1663b-0d9e-4a02-bdc0-bed7f1719d3b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_FifthAnniversaryMember_374a08f6-8ad0-4ab9-b354-95dbfceb8554" xlink:href="arqt-20210930.xsd#arqt_FifthAnniversaryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeaseContractualTermDomain_24e2e76f-4c1f-47b0-8a76-5ea291c6ee77" xlink:to="loc_arqt_FifthAnniversaryMember_374a08f6-8ad0-4ab9-b354-95dbfceb8554" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_SubsequentAnniversariesMember_266c693e-a3ea-4002-9946-a28b8f62de91" xlink:href="arqt-20210930.xsd#arqt_SubsequentAnniversariesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeaseContractualTermDomain_24e2e76f-4c1f-47b0-8a76-5ea291c6ee77" xlink:to="loc_arqt_SubsequentAnniversariesMember_266c693e-a3ea-4002-9946-a28b8f62de91" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://arcutis.com/role/ConvertiblePreferredStockandStockholdersEquityNarrativeDetails" xlink:type="simple" xlink:href="arqt-20210930.xsd#ConvertiblePreferredStockandStockholdersEquityNarrativeDetails"/>
  <link:definitionLink xlink:role="http://arcutis.com/role/ConvertiblePreferredStockandStockholdersEquityNarrativeDetails" xlink:type="extended" id="i957a5e3bbccd48a781a4f15a4b0928de_ConvertiblePreferredStockandStockholdersEquityNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_1137835f-d7b9-4fb3-848b-440b49b1716a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_785cf846-2651-4999-ae9f-ba40167a4c18" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_1137835f-d7b9-4fb3-848b-440b49b1716a" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities_785cf846-2651-4999-ae9f-ba40167a4c18" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_CommonStockVotingRightsNumberOfVotesPerShare_7c1e4f1b-5b54-49b8-936c-90ff2d472b19" xlink:href="arqt-20210930.xsd#arqt_CommonStockVotingRightsNumberOfVotesPerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_1137835f-d7b9-4fb3-848b-440b49b1716a" xlink:to="loc_arqt_CommonStockVotingRightsNumberOfVotesPerShare_7c1e4f1b-5b54-49b8-936c-90ff2d472b19" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsPayableCurrentAndNoncurrent_613f4a1a-8c70-4d2f-ad03-0da693375a4b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DividendsPayableCurrentAndNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_1137835f-d7b9-4fb3-848b-440b49b1716a" xlink:to="loc_us-gaap_DividendsPayableCurrentAndNoncurrent_613f4a1a-8c70-4d2f-ad03-0da693375a4b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_7409b15e-0ff7-40fe-be2d-99b2c5469f68" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_1137835f-d7b9-4fb3-848b-440b49b1716a" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_7409b15e-0ff7-40fe-be2d-99b2c5469f68" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_a0fc01f1-1d0e-4d73-abf6-d0d6a8d68f37" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_1137835f-d7b9-4fb3-848b-440b49b1716a" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_a0fc01f1-1d0e-4d73-abf6-d0d6a8d68f37" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquitySharesAuthorized_63c8e0ba-e430-46e7-a3f4-2637433325da" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TemporaryEquitySharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_1137835f-d7b9-4fb3-848b-440b49b1716a" xlink:to="loc_us-gaap_TemporaryEquitySharesAuthorized_63c8e0ba-e430-46e7-a3f4-2637433325da" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TemporaryEquityParOrStatedValuePerShare_c1acfd46-4a9a-420d-b5ca-d3e37e18b6bd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TemporaryEquityParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_1137835f-d7b9-4fb3-848b-440b49b1716a" xlink:to="loc_us-gaap_TemporaryEquityParOrStatedValuePerShare_c1acfd46-4a9a-420d-b5ca-d3e37e18b6bd" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesOutstanding_e01b07d2-ae4a-42e5-8a9c-794ae4f29af0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PreferredStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_1137835f-d7b9-4fb3-848b-440b49b1716a" xlink:to="loc_us-gaap_PreferredStockSharesOutstanding_e01b07d2-ae4a-42e5-8a9c-794ae4f29af0" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_9b331372-3b56-4b65-a0a4-a3de46c225c6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_1137835f-d7b9-4fb3-848b-440b49b1716a" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_9b331372-3b56-4b65-a0a4-a3de46c225c6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_ffe58f0d-57cd-4fe6-9126-4cb323c9ac38" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_9b331372-3b56-4b65-a0a4-a3de46c225c6" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_ffe58f0d-57cd-4fe6-9126-4cb323c9ac38" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_ffe58f0d-57cd-4fe6-9126-4cb323c9ac38_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_ffe58f0d-57cd-4fe6-9126-4cb323c9ac38" xlink:to="loc_us-gaap_EquityComponentDomain_ffe58f0d-57cd-4fe6-9126-4cb323c9ac38_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_d603b054-9a93-45a3-b44b-23014058551b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_ffe58f0d-57cd-4fe6-9126-4cb323c9ac38" xlink:to="loc_us-gaap_EquityComponentDomain_d603b054-9a93-45a3-b44b-23014058551b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_8e1e74b7-3556-47a9-a77f-9be67295bc6b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_d603b054-9a93-45a3-b44b-23014058551b" xlink:to="loc_us-gaap_CommonStockMember_8e1e74b7-3556-47a9-a77f-9be67295bc6b" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://arcutis.com/role/ConvertiblePreferredStockandStockholdersEquitySharesofCommonStockforIssuanceDetails" xlink:type="simple" xlink:href="arqt-20210930.xsd#ConvertiblePreferredStockandStockholdersEquitySharesofCommonStockforIssuanceDetails"/>
  <link:definitionLink xlink:role="http://arcutis.com/role/ConvertiblePreferredStockandStockholdersEquitySharesofCommonStockforIssuanceDetails" xlink:type="extended" id="id03404e36fa440c89533ca0d704aa6e2_ConvertiblePreferredStockandStockholdersEquitySharesofCommonStockforIssuanceDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_c27ceb52-6515-4b61-85e5-ca44d25dd566" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_ffcdd066-f17d-4a1c-815b-3995cbf330d1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_c27ceb52-6515-4b61-85e5-ca44d25dd566" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_ffcdd066-f17d-4a1c-815b-3995cbf330d1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_2784e533-5b73-402d-80ae-ad2cd8a0e989" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_c27ceb52-6515-4b61-85e5-ca44d25dd566" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_2784e533-5b73-402d-80ae-ad2cd8a0e989" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_bbf388ce-96a6-4f61-9319-af4b3116030a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_2784e533-5b73-402d-80ae-ad2cd8a0e989" xlink:to="loc_us-gaap_AwardTypeAxis_bbf388ce-96a6-4f61-9319-af4b3116030a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_bbf388ce-96a6-4f61-9319-af4b3116030a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_bbf388ce-96a6-4f61-9319-af4b3116030a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_bbf388ce-96a6-4f61-9319-af4b3116030a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_edf0ae16-0d36-4105-93db-feb41ffa7eae" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_bbf388ce-96a6-4f61-9319-af4b3116030a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_edf0ae16-0d36-4105-93db-feb41ffa7eae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_StockOptionsIssuedAndOutstandingMember_687a1681-9c4b-414c-9ebe-15a6a26dc917" xlink:href="arqt-20210930.xsd#arqt_StockOptionsIssuedAndOutstandingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_edf0ae16-0d36-4105-93db-feb41ffa7eae" xlink:to="loc_arqt_StockOptionsIssuedAndOutstandingMember_687a1681-9c4b-414c-9ebe-15a6a26dc917" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_StockAwardsFutureGrantMember_624bcf3e-f031-4f29-b4d4-36374d745124" xlink:href="arqt-20210930.xsd#arqt_StockAwardsFutureGrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_edf0ae16-0d36-4105-93db-feb41ffa7eae" xlink:to="loc_arqt_StockAwardsFutureGrantMember_624bcf3e-f031-4f29-b4d4-36374d745124" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_e199c13e-7c7e-43dc-aca8-53b27183c945" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_edf0ae16-0d36-4105-93db-feb41ffa7eae" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_e199c13e-7c7e-43dc-aca8-53b27183c945" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://arcutis.com/role/StockBasedCompensationNarrativeDetails" xlink:type="simple" xlink:href="arqt-20210930.xsd#StockBasedCompensationNarrativeDetails"/>
  <link:definitionLink xlink:role="http://arcutis.com/role/StockBasedCompensationNarrativeDetails" xlink:type="extended" id="ied1c3512a3cc4909b53570591ccbcc7d_StockBasedCompensationNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9f605c0d-c7ca-4d03-a81d-9d0707ef0595" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_c613d7e5-da25-4571-88dc-cb3c37a26a03" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9f605c0d-c7ca-4d03-a81d-9d0707ef0595" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_c613d7e5-da25-4571-88dc-cb3c37a26a03" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_ShareBasedCompensationArrangementByShareBasedPaymentAwardIncreaseInNumberOfSharesAvailableForGrantPercentageOfSharesOutstanding_f0e76113-5b80-49b0-a874-2c5e13903cbd" xlink:href="arqt-20210930.xsd#arqt_ShareBasedCompensationArrangementByShareBasedPaymentAwardIncreaseInNumberOfSharesAvailableForGrantPercentageOfSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9f605c0d-c7ca-4d03-a81d-9d0707ef0595" xlink:to="loc_arqt_ShareBasedCompensationArrangementByShareBasedPaymentAwardIncreaseInNumberOfSharesAvailableForGrantPercentageOfSharesOutstanding_f0e76113-5b80-49b0-a874-2c5e13903cbd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_ShareBasedCompensationArrangementByShareBasedPaymentAwardBaseNumberOfOptionsIssuable_3b2fd361-23aa-4d98-946e-8b4f488b6851" xlink:href="arqt-20210930.xsd#arqt_ShareBasedCompensationArrangementByShareBasedPaymentAwardBaseNumberOfOptionsIssuable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9f605c0d-c7ca-4d03-a81d-9d0707ef0595" xlink:to="loc_arqt_ShareBasedCompensationArrangementByShareBasedPaymentAwardBaseNumberOfOptionsIssuable_3b2fd361-23aa-4d98-946e-8b4f488b6851" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_a38c8ce4-b353-4ef4-8acd-f1bcc49abc98" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9f605c0d-c7ca-4d03-a81d-9d0707ef0595" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_a38c8ce4-b353-4ef4-8acd-f1bcc49abc98" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_918dddbc-fe43-4807-8988-af3ba196b706" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9f605c0d-c7ca-4d03-a81d-9d0707ef0595" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_918dddbc-fe43-4807-8988-af3ba196b706" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_346ef33d-41b5-4816-924c-afeaf616c7fd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9f605c0d-c7ca-4d03-a81d-9d0707ef0595" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_346ef33d-41b5-4816-924c-afeaf616c7fd" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_3908872b-34b7-4efb-8db9-995c4d26e77a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9f605c0d-c7ca-4d03-a81d-9d0707ef0595" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_3908872b-34b7-4efb-8db9-995c4d26e77a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_79161a94-f64e-4996-9a2c-a505264070ec" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9f605c0d-c7ca-4d03-a81d-9d0707ef0595" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_79161a94-f64e-4996-9a2c-a505264070ec" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_dd1883fb-b00c-498d-aa40-d329a3523ffb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9f605c0d-c7ca-4d03-a81d-9d0707ef0595" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_dd1883fb-b00c-498d-aa40-d329a3523ffb" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_daddb32b-7ffb-423e-ae0f-8f947eb29a4b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_28170db7-9e2e-4988-98b5-a1c9513db7d1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9f605c0d-c7ca-4d03-a81d-9d0707ef0595" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_28170db7-9e2e-4988-98b5-a1c9513db7d1" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_44728a8a-7b3e-4c36-83c1-78746fc345cf" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9f605c0d-c7ca-4d03-a81d-9d0707ef0595" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_44728a8a-7b3e-4c36-83c1-78746fc345cf" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPlanModificationIncrementalCompensationCost_49adeec6-b304-41b7-b41b-768513a043a0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPlanModificationIncrementalCompensationCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9f605c0d-c7ca-4d03-a81d-9d0707ef0595" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPlanModificationIncrementalCompensationCost_49adeec6-b304-41b7-b41b-768513a043a0" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_d92d3077-2b5f-4c66-953a-7fa4b006aa78" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9f605c0d-c7ca-4d03-a81d-9d0707ef0595" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_d92d3077-2b5f-4c66-953a-7fa4b006aa78" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredCompensationSharebasedArrangementsLiabilityCurrentAndNoncurrent_31c2bff6-b977-46d0-9446-e413608fa221" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredCompensationSharebasedArrangementsLiabilityCurrentAndNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9f605c0d-c7ca-4d03-a81d-9d0707ef0595" xlink:to="loc_us-gaap_DeferredCompensationSharebasedArrangementsLiabilityCurrentAndNoncurrent_31c2bff6-b977-46d0-9446-e413608fa221" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredCompensationShareBasedArrangementsLiabilityCurrent_c36ef5eb-89b4-419b-a8e0-abbeb19b66cb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredCompensationShareBasedArrangementsLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9f605c0d-c7ca-4d03-a81d-9d0707ef0595" xlink:to="loc_us-gaap_DeferredCompensationShareBasedArrangementsLiabilityCurrent_c36ef5eb-89b4-419b-a8e0-abbeb19b66cb" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredCompensationSharebasedArrangementsLiabilityClassifiedNoncurrent_4f13ef88-2074-4b5a-ac37-8bac5b6fbfee" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredCompensationSharebasedArrangementsLiabilityClassifiedNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9f605c0d-c7ca-4d03-a81d-9d0707ef0595" xlink:to="loc_us-gaap_DeferredCompensationSharebasedArrangementsLiabilityClassifiedNoncurrent_4f13ef88-2074-4b5a-ac37-8bac5b6fbfee" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod_4ba4800b-6a18-40fa-b623-34852435372c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9f605c0d-c7ca-4d03-a81d-9d0707ef0595" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod_4ba4800b-6a18-40fa-b623-34852435372c" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_31b3e6a7-0123-4d87-bac9-4fec9f25626b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9f605c0d-c7ca-4d03-a81d-9d0707ef0595" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_31b3e6a7-0123-4d87-bac9-4fec9f25626b" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesVestingOnAMonthlyBasis_eabe7f52-bb55-4562-a290-30bd3ece0a91" xlink:href="arqt-20210930.xsd#arqt_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesVestingOnAMonthlyBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9f605c0d-c7ca-4d03-a81d-9d0707ef0595" xlink:to="loc_arqt_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesVestingOnAMonthlyBasis_eabe7f52-bb55-4562-a290-30bd3ece0a91" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_f285e5c4-2767-4ea7-8284-53a9b13f4067" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9f605c0d-c7ca-4d03-a81d-9d0707ef0595" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_f285e5c4-2767-4ea7-8284-53a9b13f4067" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_dbcfa42f-463e-4923-8f7b-59d1bd0a21d6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9f605c0d-c7ca-4d03-a81d-9d0707ef0595" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_dbcfa42f-463e-4923-8f7b-59d1bd0a21d6" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_ShareBasedCompensationArrangementByShareBasedPaymentAwardBaseAmountOfCommonStockSharesAuthorized_1673a525-d37e-4718-acfc-5011e464bf7b" xlink:href="arqt-20210930.xsd#arqt_ShareBasedCompensationArrangementByShareBasedPaymentAwardBaseAmountOfCommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9f605c0d-c7ca-4d03-a81d-9d0707ef0595" xlink:to="loc_arqt_ShareBasedCompensationArrangementByShareBasedPaymentAwardBaseAmountOfCommonStockSharesAuthorized_1673a525-d37e-4718-acfc-5011e464bf7b" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_ShareBasedCompensationArrangementByShareBasedPaymentAwardIncreaseInNumberOfSharesAvailableForGrantPercentageOfSharesOutstanding_89a63902-189c-4e82-9607-bfcd6f515d03" xlink:href="arqt-20210930.xsd#arqt_ShareBasedCompensationArrangementByShareBasedPaymentAwardIncreaseInNumberOfSharesAvailableForGrantPercentageOfSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumSharesIssuable_3743e846-3976-4fb3-9775-61745bace9dd" xlink:href="arqt-20210930.xsd#arqt_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumSharesIssuable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9f605c0d-c7ca-4d03-a81d-9d0707ef0595" xlink:to="loc_arqt_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumSharesIssuable_3743e846-3976-4fb3-9775-61745bace9dd" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_495adbfa-2307-42d1-a1e4-1a4ab9430802" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9f605c0d-c7ca-4d03-a81d-9d0707ef0595" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_495adbfa-2307-42d1-a1e4-1a4ab9430802" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_bf65837b-15bb-42e1-b1a8-619f6e8d9cfc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9f605c0d-c7ca-4d03-a81d-9d0707ef0595" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_bf65837b-15bb-42e1-b1a8-619f6e8d9cfc" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_c925db88-da7d-408a-aad7-36da5024915b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_9f605c0d-c7ca-4d03-a81d-9d0707ef0595" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_c925db88-da7d-408a-aad7-36da5024915b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_606723b3-0616-44c3-ab4e-d00f55d7e69d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_c925db88-da7d-408a-aad7-36da5024915b" xlink:to="loc_us-gaap_PlanNameAxis_606723b3-0616-44c3-ab4e-d00f55d7e69d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_606723b3-0616-44c3-ab4e-d00f55d7e69d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_606723b3-0616-44c3-ab4e-d00f55d7e69d" xlink:to="loc_us-gaap_PlanNameDomain_606723b3-0616-44c3-ab4e-d00f55d7e69d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_ce551149-d859-47f7-8226-438e01739324" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_606723b3-0616-44c3-ab4e-d00f55d7e69d" xlink:to="loc_us-gaap_PlanNameDomain_ce551149-d859-47f7-8226-438e01739324" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_A2020EquityIncentivePlanMember_0cdeae58-2369-49b0-b738-4105b29ad569" xlink:href="arqt-20210930.xsd#arqt_A2020EquityIncentivePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_ce551149-d859-47f7-8226-438e01739324" xlink:to="loc_arqt_A2020EquityIncentivePlanMember_0cdeae58-2369-49b0-b738-4105b29ad569" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_A2017EquityIncentivePlanMember_51964a4e-7ce1-4935-a263-3eae3de372b6" xlink:href="arqt-20210930.xsd#arqt_A2017EquityIncentivePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_ce551149-d859-47f7-8226-438e01739324" xlink:to="loc_arqt_A2017EquityIncentivePlanMember_51964a4e-7ce1-4935-a263-3eae3de372b6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_7154d28a-bb45-4ff1-82ec-969145fe1ee9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_c925db88-da7d-408a-aad7-36da5024915b" xlink:to="loc_us-gaap_AwardTypeAxis_7154d28a-bb45-4ff1-82ec-969145fe1ee9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7154d28a-bb45-4ff1-82ec-969145fe1ee9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_7154d28a-bb45-4ff1-82ec-969145fe1ee9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7154d28a-bb45-4ff1-82ec-969145fe1ee9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2db2b2a1-7757-4b6d-9e1e-2398295ef966" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_7154d28a-bb45-4ff1-82ec-969145fe1ee9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2db2b2a1-7757-4b6d-9e1e-2398295ef966" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_aa4f89b9-3927-4d15-b45f-c39b15923e02" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2db2b2a1-7757-4b6d-9e1e-2398295ef966" xlink:to="loc_us-gaap_EmployeeStockOptionMember_aa4f89b9-3927-4d15-b45f-c39b15923e02" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_74ce4e7b-8a16-4833-bba5-59ed443bf3b5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2db2b2a1-7757-4b6d-9e1e-2398295ef966" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_74ce4e7b-8a16-4833-bba5-59ed443bf3b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_5eca9aaf-26cf-4c86-9d94-c9b4ab3a5730" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2db2b2a1-7757-4b6d-9e1e-2398295ef966" xlink:to="loc_us-gaap_RestrictedStockMember_5eca9aaf-26cf-4c86-9d94-c9b4ab3a5730" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_ServiceConditionRestrictedStockMember_ba51f4fb-7704-4ad8-88d4-7d68431b0045" xlink:href="arqt-20210930.xsd#arqt_ServiceConditionRestrictedStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2db2b2a1-7757-4b6d-9e1e-2398295ef966" xlink:to="loc_arqt_ServiceConditionRestrictedStockMember_ba51f4fb-7704-4ad8-88d4-7d68431b0045" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_PerformanceConditionRestrictedStockMember_b6ae1505-d31e-4f75-a89f-0b5173c7bc05" xlink:href="arqt-20210930.xsd#arqt_PerformanceConditionRestrictedStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2db2b2a1-7757-4b6d-9e1e-2398295ef966" xlink:to="loc_arqt_PerformanceConditionRestrictedStockMember_b6ae1505-d31e-4f75-a89f-0b5173c7bc05" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_4a65b4d5-55e4-4a27-a01b-8f20e5725a67" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2db2b2a1-7757-4b6d-9e1e-2398295ef966" xlink:to="loc_us-gaap_EmployeeStockMember_4a65b4d5-55e4-4a27-a01b-8f20e5725a67" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockCompensationPlanMember_fc8cac4a-087e-42b7-abc8-72d358dd26ee" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockCompensationPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2db2b2a1-7757-4b6d-9e1e-2398295ef966" xlink:to="loc_us-gaap_StockCompensationPlanMember_fc8cac4a-087e-42b7-abc8-72d358dd26ee" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_280ba4bb-d376-4d5b-8480-9688fa1f9efc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_c925db88-da7d-408a-aad7-36da5024915b" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_280ba4bb-d376-4d5b-8480-9688fa1f9efc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_280ba4bb-d376-4d5b-8480-9688fa1f9efc_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_280ba4bb-d376-4d5b-8480-9688fa1f9efc" xlink:to="loc_us-gaap_EquityComponentDomain_280ba4bb-d376-4d5b-8480-9688fa1f9efc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_33651600-7c40-494b-b3f3-f05ceb10eaf6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_280ba4bb-d376-4d5b-8480-9688fa1f9efc" xlink:to="loc_us-gaap_EquityComponentDomain_33651600-7c40-494b-b3f3-f05ceb10eaf6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_210ca4d8-7f3c-481c-aef2-2bb3836732c3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_33651600-7c40-494b-b3f3-f05ceb10eaf6" xlink:to="loc_us-gaap_CommonStockMember_210ca4d8-7f3c-481c-aef2-2bb3836732c3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingAxis_c78698b7-e7e4-4a54-b4f8-2f06752a110f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_VestingAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_c925db88-da7d-408a-aad7-36da5024915b" xlink:to="loc_us-gaap_VestingAxis_c78698b7-e7e4-4a54-b4f8-2f06752a110f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_c78698b7-e7e4-4a54-b4f8-2f06752a110f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VestingAxis_c78698b7-e7e4-4a54-b4f8-2f06752a110f" xlink:to="loc_us-gaap_VestingDomain_c78698b7-e7e4-4a54-b4f8-2f06752a110f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_058b3a52-e670-4ada-a88c-2dc85760d5df" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VestingAxis_c78698b7-e7e4-4a54-b4f8-2f06752a110f" xlink:to="loc_us-gaap_VestingDomain_058b3a52-e670-4ada-a88c-2dc85760d5df" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_ad47c708-3d49-4078-9708-3d1550436f8c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationAwardTrancheOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_058b3a52-e670-4ada-a88c-2dc85760d5df" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_ad47c708-3d49-4078-9708-3d1550436f8c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_8baf325f-c55a-415e-9b0d-70fd47147291" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationAwardTrancheTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_058b3a52-e670-4ada-a88c-2dc85760d5df" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_8baf325f-c55a-415e-9b0d-70fd47147291" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://arcutis.com/role/StockBasedCompensationRestrictedStockUnitActivityDetails" xlink:type="simple" xlink:href="arqt-20210930.xsd#StockBasedCompensationRestrictedStockUnitActivityDetails"/>
  <link:definitionLink xlink:role="http://arcutis.com/role/StockBasedCompensationRestrictedStockUnitActivityDetails" xlink:type="extended" id="i3ae6b8c7a935400f900e00a74527fc07_StockBasedCompensationRestrictedStockUnitActivityDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_8f8fb4ea-23b0-4159-83ba-0216023bf9b7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_ea8f3b1b-628e-4773-a4d2-e41c78f875b2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_8f8fb4ea-23b0-4159-83ba-0216023bf9b7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_ea8f3b1b-628e-4773-a4d2-e41c78f875b2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_79227aef-9b9d-4951-80f8-e60220ca7f76" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_ea8f3b1b-628e-4773-a4d2-e41c78f875b2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_79227aef-9b9d-4951-80f8-e60220ca7f76" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_62bf900f-48ad-42a1-92fe-0cc87342f37b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_ea8f3b1b-628e-4773-a4d2-e41c78f875b2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_62bf900f-48ad-42a1-92fe-0cc87342f37b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_817b0537-cb0f-4c3d-9617-c157982c0afc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_ea8f3b1b-628e-4773-a4d2-e41c78f875b2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_817b0537-cb0f-4c3d-9617-c157982c0afc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_f622607f-d45e-4970-837f-3d9037b531d1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_ea8f3b1b-628e-4773-a4d2-e41c78f875b2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_f622607f-d45e-4970-837f-3d9037b531d1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_7fff8905-e632-4827-8f9c-8040bac895f8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_fa47bd7f-6585-43a0-8678-90fbe98dbb0a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_8f8fb4ea-23b0-4159-83ba-0216023bf9b7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_fa47bd7f-6585-43a0-8678-90fbe98dbb0a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_45103c3a-137a-4164-9d5a-a1538e908261" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_fa47bd7f-6585-43a0-8678-90fbe98dbb0a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_45103c3a-137a-4164-9d5a-a1538e908261" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_8ff9ab4c-eb1d-4df3-91d2-b732af0500ed" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_fa47bd7f-6585-43a0-8678-90fbe98dbb0a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_8ff9ab4c-eb1d-4df3-91d2-b732af0500ed" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedWeightedAverageGrantDateFairValue_51ffb0ca-2c48-44af-975a-5839cd63d0a3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_fa47bd7f-6585-43a0-8678-90fbe98dbb0a" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedWeightedAverageGrantDateFairValue_51ffb0ca-2c48-44af-975a-5839cd63d0a3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_a05342c4-d323-4811-9492-6b5ea4060511" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_fa47bd7f-6585-43a0-8678-90fbe98dbb0a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_a05342c4-d323-4811-9492-6b5ea4060511" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_e033c243-710a-4540-b965-4b0440b5a8a2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_ebd78f4b-8727-4f45-9758-58759d3b2e9f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_8f8fb4ea-23b0-4159-83ba-0216023bf9b7" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_ebd78f4b-8727-4f45-9758-58759d3b2e9f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_e82bee47-1296-45a1-97a2-21c3e3f33ddb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_ebd78f4b-8727-4f45-9758-58759d3b2e9f" xlink:to="loc_us-gaap_AwardTypeAxis_e82bee47-1296-45a1-97a2-21c3e3f33ddb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e82bee47-1296-45a1-97a2-21c3e3f33ddb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_e82bee47-1296-45a1-97a2-21c3e3f33ddb" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e82bee47-1296-45a1-97a2-21c3e3f33ddb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_a9759b16-e465-4c99-bf23-b947b6a0db1e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_e82bee47-1296-45a1-97a2-21c3e3f33ddb" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_a9759b16-e465-4c99-bf23-b947b6a0db1e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_41f3522e-97d0-4a8b-82b7-eed241bea7fc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_a9759b16-e465-4c99-bf23-b947b6a0db1e" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_41f3522e-97d0-4a8b-82b7-eed241bea7fc" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://arcutis.com/role/StockBasedCompensationStockBasedCompensationExpenseDetails" xlink:type="simple" xlink:href="arqt-20210930.xsd#StockBasedCompensationStockBasedCompensationExpenseDetails"/>
  <link:definitionLink xlink:role="http://arcutis.com/role/StockBasedCompensationStockBasedCompensationExpenseDetails" xlink:type="extended" id="i51ebdda4d68c40b2a1cb31d9f17b9556_StockBasedCompensationStockBasedCompensationExpenseDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_77140731-c4d4-48c6-ac7e-ad5201048ddc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_3ec89e3f-efe0-41a2-bbfd-267c65367b04" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_77140731-c4d4-48c6-ac7e-ad5201048ddc" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_3ec89e3f-efe0-41a2-bbfd-267c65367b04" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_33122e5d-5bcc-4e42-a5df-1c74495c6aed" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_77140731-c4d4-48c6-ac7e-ad5201048ddc" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_33122e5d-5bcc-4e42-a5df-1c74495c6aed" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_362fa9f2-dbb4-4ece-9fbe-528cb29361fa" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_33122e5d-5bcc-4e42-a5df-1c74495c6aed" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_362fa9f2-dbb4-4ece-9fbe-528cb29361fa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_362fa9f2-dbb4-4ece-9fbe-528cb29361fa_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_362fa9f2-dbb4-4ece-9fbe-528cb29361fa" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_362fa9f2-dbb4-4ece-9fbe-528cb29361fa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_7c22bcd5-2926-4598-9538-723ebafbc07d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_362fa9f2-dbb4-4ece-9fbe-528cb29361fa" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_7c22bcd5-2926-4598-9538-723ebafbc07d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_77f12302-9965-434a-9f56-51cb6e2e79fd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ResearchAndDevelopmentExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_7c22bcd5-2926-4598-9538-723ebafbc07d" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_77f12302-9965-434a-9f56-51cb6e2e79fd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpenseMember_e46d1e71-96b3-47d6-8f8a-6aefdb4c1b12" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GeneralAndAdministrativeExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_7c22bcd5-2926-4598-9538-723ebafbc07d" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpenseMember_e46d1e71-96b3-47d6-8f8a-6aefdb4c1b12" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://arcutis.com/role/StockBasedCompensationAssumptionsinCalculatingStockOptionAwardsDetails" xlink:type="simple" xlink:href="arqt-20210930.xsd#StockBasedCompensationAssumptionsinCalculatingStockOptionAwardsDetails"/>
  <link:definitionLink xlink:role="http://arcutis.com/role/StockBasedCompensationAssumptionsinCalculatingStockOptionAwardsDetails" xlink:type="extended" id="ic34f6ab409a146c6b408e79bb66d6440_StockBasedCompensationAssumptionsinCalculatingStockOptionAwardsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5551907c-8d29-43a3-b7ea-f1cc9f18d978" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_312612ee-14f7-48ae-af8b-ff236a91a01e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5551907c-8d29-43a3-b7ea-f1cc9f18d978" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_312612ee-14f7-48ae-af8b-ff236a91a01e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMinimum_e4035808-4796-46b9-9577-533bd2de54a7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMinimum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5551907c-8d29-43a3-b7ea-f1cc9f18d978" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMinimum_e4035808-4796-46b9-9577-533bd2de54a7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMaximum_c08cc662-fee3-4da2-9a27-3c762888d57d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMaximum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5551907c-8d29-43a3-b7ea-f1cc9f18d978" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMaximum_c08cc662-fee3-4da2-9a27-3c762888d57d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum_1b342ebe-2940-4e06-a5d0-ac7ec8b4b763" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5551907c-8d29-43a3-b7ea-f1cc9f18d978" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum_1b342ebe-2940-4e06-a5d0-ac7ec8b4b763" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum_5fadcab2-a10b-4970-939c-e41a4da13118" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5551907c-8d29-43a3-b7ea-f1cc9f18d978" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum_5fadcab2-a10b-4970-939c-e41a4da13118" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_f3c9bac7-556e-408d-a50c-03e7b39e37e5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5551907c-8d29-43a3-b7ea-f1cc9f18d978" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_f3c9bac7-556e-408d-a50c-03e7b39e37e5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_f55a760c-7c61-41bf-bf13-16cc64bcdf93" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_5551907c-8d29-43a3-b7ea-f1cc9f18d978" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_f55a760c-7c61-41bf-bf13-16cc64bcdf93" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_a96e683b-412d-45c1-8e90-b51959a95457" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_f55a760c-7c61-41bf-bf13-16cc64bcdf93" xlink:to="loc_srt_RangeAxis_a96e683b-412d-45c1-8e90-b51959a95457" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_a96e683b-412d-45c1-8e90-b51959a95457_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_a96e683b-412d-45c1-8e90-b51959a95457" xlink:to="loc_srt_RangeMember_a96e683b-412d-45c1-8e90-b51959a95457_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_0976b759-f5b2-403b-a157-86fdf6d06916" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_a96e683b-412d-45c1-8e90-b51959a95457" xlink:to="loc_srt_RangeMember_0976b759-f5b2-403b-a157-86fdf6d06916" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_d52b8a43-a7ce-445a-9e58-dc429c751acc" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_0976b759-f5b2-403b-a157-86fdf6d06916" xlink:to="loc_srt_MinimumMember_d52b8a43-a7ce-445a-9e58-dc429c751acc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_653a5f17-aa4c-4b26-8f36-c8848edc0c0a" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_0976b759-f5b2-403b-a157-86fdf6d06916" xlink:to="loc_srt_MaximumMember_653a5f17-aa4c-4b26-8f36-c8848edc0c0a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_eb1908c8-2c57-49c8-9cd1-eee740b9c1c9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_f55a760c-7c61-41bf-bf13-16cc64bcdf93" xlink:to="loc_us-gaap_AwardTypeAxis_eb1908c8-2c57-49c8-9cd1-eee740b9c1c9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_eb1908c8-2c57-49c8-9cd1-eee740b9c1c9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_eb1908c8-2c57-49c8-9cd1-eee740b9c1c9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_eb1908c8-2c57-49c8-9cd1-eee740b9c1c9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d2f02e4f-1805-4ac6-b2c2-dd9cf0178c55" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_eb1908c8-2c57-49c8-9cd1-eee740b9c1c9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d2f02e4f-1805-4ac6-b2c2-dd9cf0178c55" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_9fb0ddad-8225-4b75-a16c-d79432b023c9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_d2f02e4f-1805-4ac6-b2c2-dd9cf0178c55" xlink:to="loc_us-gaap_EmployeeStockOptionMember_9fb0ddad-8225-4b75-a16c-d79432b023c9" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://arcutis.com/role/NetLossPerShareDetails" xlink:type="simple" xlink:href="arqt-20210930.xsd#NetLossPerShareDetails"/>
  <link:definitionLink xlink:role="http://arcutis.com/role/NetLossPerShareDetails" xlink:type="extended" id="i73c25b1915ed4fb592fff27b11fff0b9_NetLossPerShareDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_24b259ad-798a-4d8b-a181-1037c0ad2743" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_24fdd8a4-b306-40d7-bb38-04deede2d938" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_24b259ad-798a-4d8b-a181-1037c0ad2743" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_24fdd8a4-b306-40d7-bb38-04deede2d938" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_b10d68b7-9f7e-4a4f-83dd-57ab7dd7a056" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_24b259ad-798a-4d8b-a181-1037c0ad2743" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_b10d68b7-9f7e-4a4f-83dd-57ab7dd7a056" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_12df5e3d-4313-4d9b-8a4f-d14fc4d53d7c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_b10d68b7-9f7e-4a4f-83dd-57ab7dd7a056" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_12df5e3d-4313-4d9b-8a4f-d14fc4d53d7c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_12df5e3d-4313-4d9b-8a4f-d14fc4d53d7c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_12df5e3d-4313-4d9b-8a4f-d14fc4d53d7c" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_12df5e3d-4313-4d9b-8a4f-d14fc4d53d7c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_8fae19e0-1285-444a-900a-8674de91f994" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_12df5e3d-4313-4d9b-8a4f-d14fc4d53d7c" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_8fae19e0-1285-444a-900a-8674de91f994" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_06ea9c44-0201-47ef-b52c-9f6a177cfcc6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_8fae19e0-1285-444a-900a-8674de91f994" xlink:to="loc_us-gaap_EmployeeStockOptionMember_06ea9c44-0201-47ef-b52c-9f6a177cfcc6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_arqt_EarlyExercisedOptionsMember_c5afc9b2-9ae9-45f3-8084-774ac49d8556" xlink:href="arqt-20210930.xsd#arqt_EarlyExercisedOptionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_8fae19e0-1285-444a-900a-8674de91f994" xlink:to="loc_arqt_EarlyExercisedOptionsMember_c5afc9b2-9ae9-45f3-8084-774ac49d8556" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_ba82e5a7-a864-4b2f-abdd-4919b1f55766" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_8fae19e0-1285-444a-900a-8674de91f994" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_ba82e5a7-a864-4b2f-abdd-4919b1f55766" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_732a3172-9c31-425f-89e8-e9cb06777637" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_8fae19e0-1285-444a-900a-8674de91f994" xlink:to="loc_us-gaap_EmployeeStockMember_732a3172-9c31-425f-89e8-e9cb06777637" xlink:type="arc" order="3"/>
  </link:definitionLink>
</link:linkbase>
