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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2022-10-16</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000726728</issuerCik>
        <issuerName>REALTY INCOME CORP</issuerName>
        <issuerTradingSymbol>O</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001529329</rptOwnerCik>
            <rptOwnerName>Roy Sumit</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>11995 EL CAMINO REAL</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>SAN DIEGO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>92130</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>President, CEO &amp; Director</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2022-10-16</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>9377</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>55.54</value>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>183347</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">This amount represents shares automatically withheld upon the vesting of 17,376 restricted shares of common stock on October 16, 2022, which amount is determined based upon the greater of such holder's minimum required tax withholding rate or the highest withholding rate permitted under the rules of the applicable taxing authority for tax withholding.</footnote>
        <footnote id="F2">Reflects the closing sale price of the Issuer's common stock as reported on the New York Stock Exchange on October 14, 2022.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Bianca Martinez, by Power of Attorney</signatureName>
        <signatureDate>2022-10-18</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0306
4
2022-10-16
0
0000726728
REALTY INCOME CORP
O
0001529329
Roy Sumit
11995 EL CAMINO REAL
SAN DIEGO
CA
92130
1
1
0
0
President, CEO & Director
Common Stock
2022-10-16
4
F
0
9377
55.54
D
183347
D
This amount represents shares automatically withheld upon the vesting of 17,376 restricted shares of common stock on October 16, 2022, which amount is determined based upon the greater of such holder's minimum required tax withholding rate or the highest withholding rate permitted under the rules of the applicable taxing authority for tax withholding.
Reflects the closing sale price of the Issuer's common stock as reported on the New York Stock Exchange on October 14, 2022.
/s/ Bianca Martinez, by Power of Attorney
2022-10-18