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

    <schemaVersion>X0206</schemaVersion>

    <documentType>3/A</documentType>

    <periodOfReport>2019-05-01</periodOfReport>

    <dateOfOriginalSubmission>2019-05-13</dateOfOriginalSubmission>

    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0001334978</issuerCik>
        <issuerName>Clear Channel Outdoor Holdings, Inc.</issuerName>
        <issuerTradingSymbol>CCO</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001163368</rptOwnerCik>
            <rptOwnerName>PACIFIC INVESTMENT MANAGEMENT CO LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>650 NEWPORT CENTER DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NEWPORT BEACH</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>92660</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>75011522</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See footnote</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Certain investment vehicles managed by the Reporting Person entered into trades to purchase debt securities of iHeartMedia, Inc. and certain of its direct and indirect domestic subsidiaries (collectively, the &quot;Debtors&quot;) that had not settled at the time the Debtors emerged from bankruptcy.  As a result, the sellers of the Debtors' debt securities received shares of the Issuer's common stock in exchange for such debt securities and delivered those shares to the investment vehicles managed by the Reporting Person after its original Form 3 was filed.  These deliveries occurred in August and December 2019 and May 2020 and, as a result, 820,614 shares of the Issuer's common stock were not included in the Reporting Person's original Form 3 and were not reflected in four Forms 4 filed by the Reporting Person after its original Form 3 was filed.</footnote>
        <footnote id="F2">The Reporting Person is the investment adviser to certain investment advisory clients or discretionary accounts, including two portfolios of PIMCO Funds, a Massachusetts business trust, one of which, PIMCO Funds: PIMCO Income Fund, then held 21,375,679 shares of Common Stock and warrants to purchase 29,304,094 shares of Common Stock and the other of which then held 100,201 shares of Common Stock and warrants to purchase 1,965,668 shares of Common Stock.  The Reporting Person disclaims beneficial ownership in the reported shares, except to the extent of its pecuniary interest therein.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Jason Nagler, Senior Vice President, Pacific Investment Management Company LLC</signatureName>
        <signatureDate>2020-07-31</signatureDate>
    </ownerSignature>
</ownershipDocument>
