<?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>0001222633</rptOwnerCik>
            <rptOwnerName>LENHARDT DAVID K</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>5320 EAST PALO VERDE DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>PARADISE VALLEY</rptOwnerCity>
            <rptOwnerState>AZ</rptOwnerState>
            <rptOwnerZipCode>85253</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>2300</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>154.89</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>554</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </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">The price reported is a weighted average price provided by the broker. These shares were sold in multiple transactions at prices ranging from $154.70 to $155.31. Mr. Lenhardt undertakes to provide to Casey's General Store, Inc., any security holder thereof, or the staff of the SEC, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.</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 6, 2018</signatureName>
        <signatureDate>2019-06-25</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0306
4
2019-06-21
0
0000726958
CASEYS GENERAL STORES INC
CASY
0001222633
LENHARDT DAVID K
5320 EAST PALO VERDE DRIVE
PARADISE VALLEY
AZ
85253
1
0
0
0
Common Stock
2019-06-21
4
S
0
2300
154.89
D
554
D
Restricted stock units
Common Stock
998.0
998
D
The price reported is a weighted average price provided by the broker. These shares were sold in multiple transactions at prices ranging from $154.70 to $155.31. Mr. Lenhardt undertakes to provide to Casey's General Store, Inc., any security holder thereof, or the staff of the SEC, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.
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 6, 2018
2019-06-25