<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0609</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2026-10-16</periodOfReport>

    <notSubjectToSection16>1</notSubjectToSection16>

    <issuer>
        <issuerCik>0001637873</issuerCik>
        <issuerName>ACV Auctions Inc.</issuerName>
        <issuerTradingSymbol>ACVA</issuerTradingSymbol>
        <issuerForeignTradingSymbol></issuerForeignTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001852328</rptOwnerCik>
            <rptOwnerName>Mehta Vikas</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerNonUSAddressFlag>false</rptOwnerNonUSAddressFlag>
            <rptOwnerStreet1>C/O ACV AUCTIONS INC.</rptOwnerStreet1>
            <rptOwnerStreet2>640 ELLICOTT ST., SUITE 321</rptOwnerStreet2>
            <rptOwnerCity>BUFFALO</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>14203</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Chief Operating Officer</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <aff10b5One>0</aff10b5One>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2026-10-16</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
                <footnoteId id="F2"/>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>274491</value>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1058015</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2026-10-16</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>U</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
                <footnoteId id="F3"/>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>1058015</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F3"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Pursuant to the Agreement and Plan of Merger, dated September 10, 2026 (the &quot;Merger Agreement&quot;), by and among ACV Auctions Inc. (&quot;ACV&quot;), Copart, Inc. (&quot;Copart&quot;), and Apple Merger Sub, Inc., a wholly owned subsidiary of Copart (&quot;Merger Sub&quot;), the shares of common stock of ACV that were tendered to Merger Sub prior to the expiration time of the offer were exchanged for $10.50 per share (&quot;Merger Consideration&quot;), net to the seller in cash, without interest, subject to any required withholding tax. After completion of the tender offer, pursuant to the terms of the Merger Agreement, Merger Sub merged with and into ACV (the &quot;Merger&quot;), with ACV surviving the Merger as a wholly owned subsidiary of Copart.</footnote>
        <footnote id="F2">Represents awards of performance-based restricted stock units which were deemed to meet the applicable performance-based vesting conditions at target performance levels.</footnote>
        <footnote id="F3">Pursuant to the terms of the Merger Agreement, (i) each outstanding restricted stock unit award corresponding to shares of ACV common stock (each, an &quot;ACV RSU&quot;) held by the Reporting Person was assumed by Copart and converted into a time-based restricted stock unit award covering a number of shares of Copart common stock determined by multiplying the number of shares of ACV common stock then subject to the ACV RSU by the Exchange Ratio (as defined in the Merger Agreement), with any fractional shares rounded to the nearest whole share and (ii) each outstanding performance-based restricted stock unit award corresponding to shares of ACV common stock (each, an &quot;ACV PSU&quot;) held by the Reporting Person was assumed by Copart and converted into a time-based restricted stock unit award covering a number of shares of Copart common stock determined by multiplying the number of shares of ACV common stock then subject to the ACV PSU (determined based on target performance) by the Exchange Ratio.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Michelle Webb, Attorney-in-Fact</signatureName>
        <signatureDate>2026-10-19</signatureDate>
    </ownerSignature>
</ownershipDocument>
