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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2019-06-21</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000726958</issuerCik>
        <issuerName>CASEYS GENERAL STORES INC</issuerName>
        <issuerTradingSymbol>CASY</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001397796</rptOwnerCik>
            <rptOwnerName>Horak H Lynn</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>504 GRAND OAKS DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>WEST DES MOINES</rptOwnerCity>
            <rptOwnerState>IA</rptOwnerState>
            <rptOwnerZipCode>50265</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>2019-06-21</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>3000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>154.55</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>12608</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>200</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Consisting of shares held in a separately managed IRA account</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Restricted stock units</value>
                <footnoteId id="F2"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F3"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F2"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>998.0</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>998</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Includes 125 shares acquired through dividend reinvestment plan (&quot;DRIP&quot;) from time to time, including certain shares acquired through the DRIP that were inadvertently omitted from the reporting person's previous Section 16 reports.</footnote>
        <footnote id="F2">Non-employee director equity compensation pursuant to terms and conditions of 2018 Stock Incentive Plan. This award will vest in full on the date of Casey's 2019 annual shareholders' meeting.</footnote>
        <footnote id="F3">Each restricted stock unit represents the right to receive, following vesting, one share of Common Stock.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Scott Faber, under Power of Attorney dated March 3, 2017</signatureName>
        <signatureDate>2019-06-25</signatureDate>
    </ownerSignature>
</ownershipDocument>
[{"type": "4", "sequence": "1", "filename": "wf-form4_156149009811243.xml", "description": "FORM 4", "secsgml_size_bytes": 5734}]
X0306
4
2019-06-21
0
0000726958
CASEYS GENERAL STORES INC
CASY
0001397796
Horak H Lynn
504 GRAND OAKS DRIVE
WEST DES MOINES
IA
50265
1
0
0
0
Common Stock
2019-06-21
4
S
0
3000
154.55
D
12608
D
Common Stock
200
I
Consisting of shares held in a separately managed IRA account
Restricted stock units
Common Stock
998.0
998
D
Includes 125 shares acquired through dividend reinvestment plan ("DRIP") from time to time, including certain shares acquired through the DRIP that were inadvertently omitted from the reporting person's previous Section 16 reports.
Non-employee director equity compensation pursuant to terms and conditions of 2018 Stock Incentive Plan. This award will vest in full on the date of Casey's 2019 annual shareholders' meeting.
Each restricted stock unit represents the right to receive, following vesting, one share of Common Stock.
Scott Faber, under Power of Attorney dated March 3, 2017
2019-06-25