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

    <schemaVersion>X0203</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2011-06-08</periodOfReport>

    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0000764579</issuerCik>
        <issuerName>CAS MEDICAL SYSTEMS INC</issuerName>
        <issuerTradingSymbol>CASM</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001369574</rptOwnerCik>
            <rptOwnerName>Thomas, McNerney &amp; Partners II L.P.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>60 SOUTH 6TH STREET</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 3620</rptOwnerStreet2>
            <rptOwnerCity>MINNEAPOLIS</rptOwnerCity>
            <rptOwnerState>MN</rptOwnerState>
            <rptOwnerZipCode>55402</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001505930</rptOwnerCik>
            <rptOwnerName>Thomas, McNerney &amp; Partners II, LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>60 SOUTH 6TH STREET</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 3620</rptOwnerStreet2>
            <rptOwnerCity>MINNEAPOLIS</rptOwnerCity>
            <rptOwnerState>MN</rptOwnerState>
            <rptOwnerZipCode>55402</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001383035</rptOwnerCik>
            <rptOwnerName>TMP Associates II LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>60 SOUTH 6TH STREET</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 3620</rptOwnerStreet2>
            <rptOwnerCity>MINNEAPOLIS</rptOwnerCity>
            <rptOwnerState>MN</rptOwnerState>
            <rptOwnerZipCode>55402</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001436054</rptOwnerCik>
            <rptOwnerName>TMP Nominee II, LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>60 SOUTH 6TH STREET</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 3620</rptOwnerStreet2>
            <rptOwnerCity>MINNEAPOLIS</rptOwnerCity>
            <rptOwnerState>MN</rptOwnerState>
            <rptOwnerZipCode>55402</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable></nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Series A Convertible Preferred Stock, $0.001 Par Value</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>2.82</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2011-06-09</value>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F4"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <footnoteId id="F4"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F1"/>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Series A Convertible Preferred Stock, $0.001 Par Value</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>2.82</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2011-06-09</value>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F4"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <footnoteId id="F4"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F2"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By TMP Nominee II, LLC</value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Series A Convertible Preferred Stock, $0.001 Par Value</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>2.82</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2011-06-09</value>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F4"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <footnoteId id="F4"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                    <footnoteId id="F3"/>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By TMP Associates II, LP</value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">See Exhibit 99.1, Note 1.</footnote>
        <footnote id="F2">See Exhibit 99.1, Note 2.</footnote>
        <footnote id="F3">See Exhibit 99.1, Note 3.</footnote>
        <footnote id="F4">See Exhibit 99.1, Note 4.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>THOMAS, MCNERNEY &amp; PARTNERS II, L.P., By:  Thomas, McNerney &amp; Partners II, LLC, its General Partner, By:  /s/ James E. Thomas, Manager</signatureName>
        <signatureDate>2011-06-13</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>THOMAS, MCNERNEY &amp; PARTNERS II, LLC, By: /s/ James E. Thomas, Manager</signatureName>
        <signatureDate>2011-06-13</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>TMP ASSOCIATES II, L.P., By:  Thomas, McNerney &amp; Partners II, LLC, its General Partner, By:  /s/ James E. Thomas, Manager</signatureName>
        <signatureDate>2011-06-13</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>TMP NOMINEE II, LLC, By: /s/ James E. Thomas, Manager, By: /s/ Peter H. McNerney, Manager</signatureName>
        <signatureDate>2011-06-13</signatureDate>
    </ownerSignature>
</ownershipDocument>
