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<!-- DFIN ActiveDisclosure(SM) XBRL Schema Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date      : 2021-03-01T13:25:57.6751243+00:00 -->
<!-- Version            : 5.0.1.571 -->
<!-- Package ID         : 6d9810b9230d4ce4b0887aa5cab95ca7 -->
<!-- Copyright (c) 2021 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
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  <xsd:element id="arct_DevelopmentAndOptionAgreementDate" name="DevelopmentAndOptionAgreementDate" type="xbrli:gYearMonthItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arct_ExpirationOfIntialTerm" name="ExpirationOfIntialTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arct_OptionToExtendInitialTermOnAnAnnualBasis" name="OptionToExtendInitialTermOnAnAnnualBasis" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arct_CollaborationAgreementTerminationDate" name="CollaborationAgreementTerminationDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arct_PercentageOfGlobalRightsReassumed" name="PercentageOfGlobalRightsReassumed" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arct_CollaborationAgreementSettlementObligationSettledAmount" name="CollaborationAgreementSettlementObligationSettledAmount" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arct_PotentialTargetLicenseTerm" name="PotentialTargetLicenseTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arct_AdjustmentsToTransactionPrice" name="AdjustmentsToTransactionPrice" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arct_UpfrontFeeReceived" name="UpfrontFeeReceived" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="arct_CollaborationAgreementContractualTerm" name="CollaborationAgreementContractualTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arct_PercentageOfCashPaymentsReceived" name="PercentageOfCashPaymentsReceived" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arct_PercentageOfConsiderationReceived" name="PercentageOfConsiderationReceived" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arct_SublicenseRevenueRecognition" name="SublicenseRevenueRecognition" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arct_RevenueRelatedToAmortizationOfUpfrontPayment" name="RevenueRelatedToAmortizationOfUpfrontPayment" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arct_NumberOfReservedVaccineDoses" name="NumberOfReservedVaccineDoses" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="arct_AccruedCysticFibrosisFoundationLiability" name="AccruedCysticFibrosisFoundationLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arct_AccruedSingaporeEconomicDevelopmentBoardLiability" name="AccruedSingaporeEconomicDevelopmentBoardLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arct_ClinicalAccrualsCurrent" name="ClinicalAccrualsCurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arct_OtherAccruedResearchAndDevelopmentExpenses" name="OtherAccruedResearchAndDevelopmentExpenses" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arct_LoanAndSecurityAgreementMember" name="LoanAndSecurityAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="arct_WesternAllianceBankMember" name="WesternAllianceBankMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="arct_RoyaltyPaymentPercentage" name="RoyaltyPaymentPercentage" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arct_PercentageRequiredToBeMaintainedInConsolidatedUnrestrictedCash" name="PercentageRequiredToBeMaintainedInConsolidatedUnrestrictedCash" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arct_DebtInstrumentInterestOnlyPaymentExtendedMaturityDate" name="DebtInstrumentInterestOnlyPaymentExtendedMaturityDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arct_WarrantFeePayable" name="WarrantFeePayable" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arct_PrepaymentFeePercentage" name="PrepaymentFeePercentage" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arct_EquityPurchaseAgreementMember" name="EquityPurchaseAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="arct_CollaborationPartnerExpandedUltragenyxMember" name="CollaborationPartnerExpandedUltragenyxMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="arct_UnvestedRestrictedOrdinarySharesMember" name="UnvestedRestrictedOrdinarySharesMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="arct_SharesIssuedPricePerShareOfferingOne" name="SharesIssuedPricePerShareOfferingOne" type="num:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="arct_SharesIssuedPricePerShareOfferingTwo" name="SharesIssuedPricePerShareOfferingTwo" type="num:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="arct_SharesIssuedPricePerShareOfferingThree" name="SharesIssuedPricePerShareOfferingThree" type="num:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="arct_ProceedsFromIssuanceOfCommonStockGross" name="ProceedsFromIssuanceOfCommonStockGross" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="arct_CommonStockSharesRestrictedFromSellingPeriodSubsequentToIssuanceDate" name="CommonStockSharesRestrictedFromSellingPeriodSubsequentToIssuanceDate" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arct_ProceedsFromIssuanceOfCommonStockOnOptionExercise" name="ProceedsFromIssuanceOfCommonStockOnOptionExercise" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="arct_TwoThousandNineteenOmnibusIncentivePlanMember" name="TwoThousandNineteenOmnibusIncentivePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="arct_TwoThousandTwentyEmployeeStockPurchasePlanMember" name="TwoThousandTwentyEmployeeStockPurchasePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="arct_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumDiscountedPurchasePrice" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumDiscountedPurchasePrice" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arct_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumDiscountedAmount" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumDiscountedAmount" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arct_MaximumPercentageOfLikelihoodOfUncertainTaxPositionRecognizedToBeSustained" name="MaximumPercentageOfLikelihoodOfUncertainTaxPositionRecognizedToBeSustained" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arct_IncomeTaxesLineItems" name="IncomeTaxesLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="arct_IncomeTaxesTable" name="IncomeTaxesTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="arct_DeferredTaxAssetsLeaseLiability" name="DeferredTaxAssetsLeaseLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="arct_DeferredTaxLiabilitiesPropertyPlantAndEquipmentAndIntangibleAssets" name="DeferredTaxLiabilitiesPropertyPlantAndEquipmentAndIntangibleAssets" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arct_DeferredTaxLiabilitiesRightOfUseAsset" name="DeferredTaxLiabilitiesRightOfUseAsset" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arct_OperatingLossCarryforwardsExpirationYear" name="OperatingLossCarryforwardsExpirationYear" type="xbrli:gYearItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arct_TaxCreditCarryforwardExpirationYear" name="TaxCreditCarryforwardExpirationYear" type="xbrli:gYearItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arct_NetOperatingLossAndResearchAndDevelopmentCreditCarryforwardsLimitationsOnCumulativeChangeInOwnershipMinimumPercentage" name="NetOperatingLossAndResearchAndDevelopmentCreditCarryforwardsLimitationsOnCumulativeChangeInOwnershipMinimumPercentage" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arct_NetOperatingLossAndResearchAndDevelopmentCreditCarryforwardsLimitationsOnCumulativeChangeInOwnershipPeriod" name="NetOperatingLossAndResearchAndDevelopmentCreditCarryforwardsLimitationsOnCumulativeChangeInOwnershipPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arct_EffectiveIncomeTaxRateReconciliationUncertainTaxPosition" name="EffectiveIncomeTaxRateReconciliationUncertainTaxPosition" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arct_ContraResearchAndDevelopmentExpense" name="ContraResearchAndDevelopmentExpense" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="arct_ContraExpenseRemainingAmountIncludedInAccruedExpenses" name="ContraExpenseRemainingAmountIncludedInAccruedExpenses" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arct_CommitmentAndContingenciesLineItems" name="CommitmentAndContingenciesLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="arct_CommitmentAndContingenciesTable" name="CommitmentAndContingenciesTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="arct_COVIDNineteenVaccineDevelopmentMember" name="COVIDNineteenVaccineDevelopmentMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="arct_LUNARCFMember" name="LUNARCFMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="arct_UnderlyingAgreementMember" name="UnderlyingAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="arct_CysticFibrosisFoundationTherapeuticsIncMember" name="CysticFibrosisFoundationTherapeuticsIncMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="arct_DisbursementPeriodOneMember" name="DisbursementPeriodOneMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="arct_DisbursementPeriodTwoMember" name="DisbursementPeriodTwoMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="arct_DisbursementPeriodThreeMember" name="DisbursementPeriodThreeMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="arct_DisbursementPeriodFourMember" name="DisbursementPeriodFourMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="arct_OctoberTwoThousandAndSeventeenLeaseAmendmentMember" name="OctoberTwoThousandAndSeventeenLeaseAmendmentMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="arct_GrantReceivableNumberOfInstallments" name="GrantReceivableNumberOfInstallments" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arct_PaymentReceivedFirstInstallment" name="PaymentReceivedFirstInstallment" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="arct_RecognizedContraExpense" name="RecognizedContraExpense" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arct_PaymentsForMatchingFundsForRemainingBudgetedCosts" name="PaymentsForMatchingFundsForRemainingBudgetedCosts" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arct_DisbursedAmountUponExecutionOfAmendment" name="DisbursedAmountUponExecutionOfAmendment" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arct_DisbursementAmountPayableUponAchievementOfProjectGoal" name="DisbursementAmountPayableUponAchievementOfProjectGoal" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arct_FinalPaymentOfDisbursementAmount" name="FinalPaymentOfDisbursementAmount" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arct_DisbursementPaymentUponAchievementOfRequiredManufacturingPracticesAndINDApplication" name="DisbursementPaymentUponAchievementOfRequiredManufacturingPracticesAndINDApplication" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arct_ContraExpenseIncludedInResearchAndDevelopmentExpense" name="ContraExpenseIncludedInResearchAndDevelopmentExpense" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="arct_MonthlyBaseRent" name="MonthlyBaseRent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="arct_AlexionPharmaceuticalsIncMember" name="AlexionPharmaceuticalsIncMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
</xsd:schema>
