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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2017-04-24</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000017313</issuerCik>
        <issuerName>CAPITAL SOUTHWEST CORP</issuerName>
        <issuerTradingSymbol>CSWC</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001413614</rptOwnerCik>
            <rptOwnerName>Armes Joseph B</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O CAPITAL SOUTHWEST CORPORATION</rptOwnerStreet1>
            <rptOwnerStreet2>5400 LYNDON B. JOHNSON FWY, SUITE 1300</rptOwnerStreet2>
            <rptOwnerCity>DALLAS</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>75240</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2017-04-24</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F2"/>
                <footnoteId id="F3"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>500</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>16.01</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>38000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $16.00 to $16.06, inclusive. The reporting person undertakes to provide to Capital Southwest Corporation, any security holder of Capital Southwest Corporation, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range.</footnote>
        <footnote id="F2">These shares were sold by the reporting person pursuant to a Rule 10b5-1 Sales Plan (the &quot;Plan&quot;) entered into on March 24, 2017. The Plan provides for the sale of shares of the issuer's Common Stock, subject to limit price provisions and an expiration date. To the extent any future sales occur pursuant to the Plan, such sales will be publicly disclosed in Form 4 filings with the Securities and Exchange Commission.</footnote>
        <footnote id="F3">Rule 10b5-1, promulgated under the Securities Exchange Act of 1934, allows executives of a company who are not in possession of material non-public information to establish pre-arranged plans to buy or sell a specified number of shares of such company's stock. Once a plan is established, the executive does not retain or exercise any discretion over sales of stock under the plan and the pre-planned trades may be executed at later dates as set forth in the plan, without regard to any subsequent material non-public information related to the company that the executive may receive.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Joseph B. Armes</signatureName>
        <signatureDate>2017-04-25</signatureDate>
    </ownerSignature>
</ownershipDocument>
