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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2018-04-06</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000937941</issuerCik>
        <issuerName>PCM, INC.</issuerName>
        <issuerTradingSymbol>PCMI</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001138532</rptOwnerCik>
            <rptOwnerName>JB CAPITAL PARTNERS LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>5 EVAN PLACE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>ARMONK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10504</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001139986</rptOwnerCik>
            <rptOwnerName>WEBER ALAN W</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>5 EVANS PLACE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>ARMONK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10504</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, $.001 par value per share</value>
            </securityTitle>
            <transactionDate>
                <value>2018-04-06</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>30500</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>8.0684</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1209619</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">This amount reported in Item 5 consists of 1,209,619 shares of Common Stock, par value $.001 per share (&quot;Common Stock&quot;), directly beneficially owned by JB Capital Partners LP (&quot;JB Capital&quot;).  In addition, Mr. Alan Weber, the general partner of JB Capital, directly beneficially owns 30,000 shares of Common Stock.  JB Capital has no beneficial ownership in the shares of Common Stock directly beneficially owned by Mr. Weber.  Mr. Weber may be deemed to beneficially own (solely for the purpose of Rule 16a-1(a)(2) under the Securities Exchange Act of 1934, as amended (&quot;Exchange Act&quot;)) certain of the shares of Common Stock set forth in Item 5 as the general partner of JB Capital.  Mr. Weber disclaims beneficial ownership of the securities reported in Item 5 except to the extent of his pecuniary interest therein, if any.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Alan W. Weber, General Partner of JB Capital Partners LP</signatureName>
        <signatureDate>2018-04-10</signatureDate>
    </ownerSignature>
</ownershipDocument>
