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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2019-01-10</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001609550</issuerCik>
        <issuerName>Inspire Medical Systems, Inc.</issuerName>
        <issuerTradingSymbol>INSP</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001474322</rptOwnerCik>
            <rptOwnerName>Tansey Casey M</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O INSPIRE MEDICAL SYSTEMS, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>9700 63RD AVENUE NORTH, SUITE 200</rptOwnerStreet2>
            <rptOwnerCity>MAPLE GROVE</rptOwnerCity>
            <rptOwnerState>MN</rptOwnerState>
            <rptOwnerZipCode>55369</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2019-01-10</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>295</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>46.58</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>201680</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1084127</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See footnote</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Represents shares of vested, restricted common stock received in lieu of cash fees pursuant to the Issuer's Non-Employee Director Compensation Policy, which shares are subject to the Reporting Person's lock-up agreement entered into in connection with the Registrant's public offering of common stock consummated on December 11, 2018.</footnote>
        <footnote id="F2">The shares are held of record by U.S. Venture Partners IX, L.P. (&quot;USVP IX&quot;). Presidio Management Group IX, LLC (&quot;PMG IX&quot;) is the general partner of USVP IX and may be deemed to have sole voting and dispositive power over the shares held by USVP IX. Casey Tansey, a director of the Registrant, and each of Irwin Federman, Steven M. Krausz, David E. Liddle, Paul A. Matteucci, Jonathan D. Root, and Philip M. Young, are managing members of PMG IX, and may be deemed to share voting and dispositive power over the shares held by USVP IX. Such persons and entities disclaim beneficial ownership of shares held by USVP IX, except to the extent of any proportionate pecuniary interest therein.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Richard Buchholz, Attorney-in-Fact</signatureName>
        <signatureDate>2019-01-14</signatureDate>
    </ownerSignature>
</ownershipDocument>
