Boundless Bio, Inc.

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

(Amendment No. )

Filed by the Registrant☒

Filed by a Party other than the Registrant☐

Check the appropriate box:

BOUNDLESS BIO, INC.

(Name of Registrant as Specified In Its Charter)

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

Payment of Filing Fee (Check all boxes that apply):

Boundless Bio, Inc.

10955 Alexandria Way, Suite 100

San Diego, CA 92121

Notice of Annual Meeting of Stockholders

To be held on June 15, 2026

Dear Stockholder:

Notice is hereby given that the 2026 Annual Meeting of Stockholders of Boundless Bio, Inc., a Delaware corporation, will be held on Monday, June 15, 2026 at 11:00 a.m. Pacific Time for the following purposes, as more fully described in the accompanying proxy statement:

to elect James Christensen, Ph.D. and Jennifer Lew as Class II directors to hold office until our 2029 annual meeting of stockholders;

to ratify the appointment of KPMG LLP as our independent registered public accounting firm for the fiscal year ending December 31, 2026; and

to transact any other business properly brought before the meeting.

The record date for the meeting is April 20, 2026. Only stockholders of record at the close of business on the record date, or their legal proxy holders, are entitled to notice of and vote at the meeting.

The meeting will be held entirely online via live audio webcast. Please visit www.proxydocs.com/BOLD for more details. You may register to attend the meeting at www.proxydocs.com/BOLD. You will be able to vote online during the meeting as further described beginning on page 2 of the accompanying proxy statement. You will need the control number included on your Notice of Internet Availability of Proxy Materials, your proxy card, or on the instructions that accompanied your proxy materials to attend the meeting. If your shares are held in “street name,” you should contact your bank, broker, or other organization that holds your shares to obtain your control number to attend the meeting. Stockholders and proxy holders attending the meeting online will be deemed to be present in person.

By Order of the Board of Directors,

/s/ Jessica Oien

Jessica Oien

Chief Legal Officer

San Diego, California

April 29, 2026

Please vote your shares promptly to help ensure the presence of a quorum at the meeting. You may vote your shares over the Internet or via a toll-free telephone number. If you received a paper copy of a proxy or voting instruction card by mail, you may submit your proxy or voting instruction card by completing, signing, dating, and returning it. Please follow the instructions in the accompanying proxy statement to vote.

TABLE OF CONTENTS

PROXY STATEMENT

2026 ANNUAL MEETING OF STOCKHOLDERS

To be held on June 15, 2026