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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2022-01-01</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001609979</rptOwnerCik>
            <rptOwnerName>Nugent Sean</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>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>See Remarks</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2022-01-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1437</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>71.59</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>12383</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 2,447 restricted shares of common stock on January 1, 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>
    </footnotes>

    <remarks>Senior Vice President, Controller, Principal Accounting Officer</remarks>

    <ownerSignature>
        <signatureName>/s/ Sean P. Nugent</signatureName>
        <signatureDate>2022-01-04</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0306
4
2022-01-01
0
0000726728
REALTY INCOME CORP
O
0001609979
Nugent Sean
11995 EL CAMINO REAL
SAN DIEGO
CA
92130
0
1
0
0
See Remarks
Common Stock
2022-01-01
4
F
0
1437
71.59
D
12383
D
This amount represents shares automatically withheld upon the vesting of 2,447 restricted shares of common stock on January 1, 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.
Senior Vice President, Controller, Principal Accounting Officer
/s/ Sean P. Nugent
2022-01-04