Exhibit 32.1

CERTIFICATIONS PURSUANT TO

18 U.S.C. SECTION 1350, AS ADOPTED PURSUANT TO

SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002

In connection with the Quarterly Report on Form 10-Q of Boundless Bio, Inc. (the Company) for the quarter ended March 31, 2024 as filed with the Securities and Exchange Commission on the date hereof (the Report), Zachary D. Hornby, President and Chief Executive Officer of the Company, and Jami Rubin, Chief Financial Officer of the Company, each hereby certifies, pursuant to 18 U.S.C. Section 1350, as adopted pursuant to Section 906 of the Sarbanes-Oxley Act of 2002, to his or her knowledge, that:

1.The Report fully complies with the requirements of Section 13(a) or Section 15(d) of the Securities Exchange Act of 1934, as amended; and

2.The information contained in the Report fairly presents, in all material respects, the financial condition and results of operations of the Company.

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Date: May 13, 2024	By:	/s/ Zachary D. Hornby
		Zachary D. Hornby
		President and Chief Executive Officer
		(Principal Executive Officer)
Date: May 13, 2024	By:	/s/ Jami Rubin
		Jami Rubin
		Chief Financial Officer
		(Principal Financial and Accounting Officer)

The foregoing certifications are being furnished solely pursuant to 18 U.S.C. Section 1350 and are not being filed as part of the Report or as a separate disclosure document.