SEC EDGAR · Insider purchase

BOLD: TANG KEVIN; TANG CAPITAL MANAGEMENT LLC reported buying shares

3 reported purchases of non-derivative securities. Code P includes open-market and private purchases; it does not establish motive.

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Became public: Oct 26, 2026
Event / period date: Oct 22, 2026
Collected: Nov 8, 2026, 6:47 PM EST

Why this is connected

This filing identifies Boundless Bio, Inc. as the issuer of the security. The company's SEC identifier connects it to BOLD.

Open the original source ↗Follow company activity in BOLD

Reported transactions

TANG KEVIN (10% owner); TANG CAPITAL MANAGEMENT LLC (10% owner)

5,800 shares · Common Stock

Oct 22, 2026 · $2.40 per share

6,234 shares · Common Stock

Oct 23, 2026 · $2.38 per share

9,606 shares · Common Stock

Oct 26, 2026 · $2.38 per share

Value calculated from the reported shares and prices: $51,619.20.

Code P covers open-market and private purchases. Grants, option exercises, and tax withholding are excluded from purchase results. Read the source footnotes for ownership and pricing details.

Source evidence

Read the source excerpt or record

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

    <schemaVersion>X0609</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2026-10-22</periodOfReport>

    <issuer>
        <issuerCik>0001782303</issuerCik>
        <issuerName>Boundless Bio, Inc.</issuerName>
        <issuerTradingSymbol>BOLD</issuerTradingSymbol>
        <issuerForeignTradingSymbol></issuerForeignTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001178579</rptOwnerCik>
            <rptOwnerName>TANG KEVIN</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerNonUSAddressFlag>false</rptOwnerNonUSAddressFlag>
            <rptOwnerStreet1>4747 EXECUTIVE DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 210</rptOwnerStreet2>
            <rptOwnerCity>SAN DIEGO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>92121</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>false</isDirector>
            <isOfficer>false</isOfficer>
            <isTenPercentOwner>true</isTenPercentOwner>
            <isOther>false</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001232621</rptOwnerCik>
            <rptOwnerName>TANG CAPITAL MANAGEMENT LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerNonUSAddressFlag>false</rptOwnerNonUSAddressFlag>
            <rptOwnerStreet1>4747 EXECUTIVE DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 210</rptOwnerStreet2>
            <rptOwnerCity>SAN DIEGO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>92121</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <aff10b5One>0</aff10b5One>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2026-10-22</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>5800</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>2.40</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2995909</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2026-10-23</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>6234</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>2.38</value>
                    <footnoteId id="F3"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>3002143</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2026-10-26</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>9606</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>2.38</value>
                    <footnoteId id="F4"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>3011749</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">The prices reported are weighted-average prices. These shares were purchased in multiple transactions at prices ranging from $2.39 to $2.40. The Reporting Persons undertake to provide the Issuer, any security holder of the Issuer or the SEC staff, upon request, all information regarding the number of shares purchased at each price within the range set forth in Footnotes 1 and 3 through 4.</footnote>
        <footnote id="F2">Tang Capital Partners, LP (&quot;TCP&quot;) beneficially owns 691,177 shares, Tang Capital Partners International, LP (&quot;TCPI&quot;) beneficially owns 950,804 shares, Tang Capital Partners III, Inc. (&quot;TCP III&quot;) beneficially owns 691,170 shares, and Tang Capital Partners IV, Inc. (&quot;TCP IV&quot;) beneficially owns 678,598 shares. Kevin Tang is the sole manager of Tang Capital Management, LLC, which is the general partner of TCP and TCPI. Kevin Tang is the sole director and Chief Executive Officer of TCP III and TCP IV, which are indirectly wholly owned by TCP. Mr. Tang has a pecuniary interest in the shares beneficially held by TCP, TCPI, TCP III and TCP IV.</footnote>
        <footnote id="F3">These shares were purchased in multiple transactions at prices ranging from $2.36 to $2.40.</footnote>
        <footnote id="F4">These shares were purchased in multiple transactions at prices ranging from $2.37 to $2.40.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Kevin Tang</signatureName>
        <signatureDate>2026-10-26</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Kevin Tang, Manager</signatureName>
        <signatureDate>2026-10-26</signatureDate>
    </ownerSignature>
</ownershipDocument>

Original documents control. Public records can be amended, and company connections may require interpretation. Nothing here is investment advice.