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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4/A</documentType>

    <periodOfReport>2019-12-05</periodOfReport>

    <dateOfOriginalSubmission>2019-12-17</dateOfOriginalSubmission>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000067625</issuerCik>
        <issuerName>MONMOUTH REAL ESTATE INVESTMENT CORP</issuerName>
        <issuerTradingSymbol>MNR</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001252977</rptOwnerCik>
            <rptOwnerName>WOLGIN STEVEN B</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>101 CRAWFORDS CORNER ROAD</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 1405</rptOwnerStreet2>
            <rptOwnerCity>HOLMDEL</rptOwnerCity>
            <rptOwnerState>NJ</rptOwnerState>
            <rptOwnerZipCode>07733</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Monmouth Real Estate Investment Corporation</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>69623.67</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Monmouth Real Estate Investment Corporation (Pref Series C)</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>6507.77</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Includes 921.46 shares acquired through dividend reinvestment as of 12/16/2019.  Since the date of his last ownership report, Mr. Wolgin transferred 9,379.38 shares of Common Stock to Lynn Wolgin pursuant to a domestic relations order.  Mr. Wolgin no longer reports as beneficially owned by him any securities owned by his ex-wife, Lynn Wolgin.  This report is being amended to clarify that the transactions were not intra-family gift transfers as previously reported.</footnote>
        <footnote id="F2">Since the date of his last ownership report, Mr. Wolgin transferred 6,005 shares of 6.125%  Series C Cumulative Redeemable Preferred Stock (&quot;Series C Preferred Stock&quot;) to Lynn Wolgin, and Lynn Wolgin transferred 500 shares of Series C Preferred Stock to Mr. Wolgin, pursuant to a domestic relations order.  Mr. Wolgin no longer reports as beneficially owned by him any securities owned by his ex-wife, Lynn Wolgin.  This report is being amended to clarify that the transactions were not intra-family gift transfers as previously reported.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Becky Coleridge, attorney in fact</signatureName>
        <signatureDate>2020-02-18</signatureDate>
    </ownerSignature>
</ownershipDocument>
