Syndax Pharmaceuticals 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:

(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 the appropriate box):

April 30, 2026

Dear Stockholder:

You are cordially invited to attend the 2026 Annual Meeting of Stockholders (the “Annual Meeting”) of Syndax Pharmaceuticals, Inc. (“Syndax”) The meeting will be held on June 10, 2026 at 12:00 p.m. EDT. The Annual Meeting will be held in a virtual meeting format only. You will not be able to attend the meeting in person. You can attend the Annual Meeting online, vote your shares electronically, and submit your questions during the Annual Meeting by visitingwww.meetnow.global/MUGAWRX.

Details regarding admission to the Annual Meeting and the business to be conducted at the Annual Meeting are more fully described in the accompanying Notice of 2026 Annual Meeting of Stockholders and proxy statement.

The agenda for the Annual Meeting includes the election of two (2) Class I directors, each to serve a three-year term, a non-binding advisory vote on the compensation paid to Syndax’s named executive officers, the ratification of the appointment of Deloitte & Touche LLP as Syndax’s independent registered public accounting firm for the fiscal year ending December 31, 2026, approval of the Company’s 2026 equity incentive plan, and approval of the Company’s 2026 employee stock purchase plan.

Under Securities and Exchange Commission rules, Syndax is providing access to the proxy materials for the Annual Meeting to stockholders via the Internet. Accordingly, you can access the proxy materials and vote your shares in advance of the Annual Meeting atwww.investorvote.com/SNDX. Instructions for accessing the proxy materials and voting are described in the attached proxy statement and in the Notice Regarding Availability of Proxy Materials that you received. Your vote is very important. Whether or not you plan to attend the Annual Meeting, please carefully review the proxy statement and then cast your vote, regardless of the number of shares you hold. If you are a stockholder of record, you may vote over the Internet, by telephone, or, if you request to receive a printed set of the proxy materials, by completing, signing, dating and mailing the accompanying proxy card in the return envelope. Submitting your vote via the Internet in advance of the Annual Meeting or by telephone or proxy card will not affect your right to vote during the Annual Meeting if you decide to attend the Annual Meeting. If your shares are held in street name (held for your account by a broker, bank or other agent), you will receive instructions from your broker, bank or other agent explaining how to vote your shares, and you will have the option to cast your vote by telephone or over the Internet in advance of the Annual Meeting if your voting instruction form from your broker, bank or other agent includes instructions and a toll-free telephone number or Internet website to do so. In any event, to be sure that your vote will be received in time, please cast your vote by your choice of available means at your earliest convenience.

We hope that you will join us on June 10, 2026 and greatly appreciate your continuing interest in Syndax.

Sincerely,

Michael A. Metzger

Chief Executive Officer

You are cordially invited to attend the Annual Meeting. Whether or not you expect to attend the Annual Meeting virtually, please vote by telephone or through the Internet, or, if you receive a paper proxy card by mail, by completing and returning the proxy card mailed to you, as promptly as possible in order to ensure your representation at the Annual Meeting. Voting instructions are provided in the Notice of Internet Availability of Proxy Materials, or, if you receive a paper proxy card by mail, the instructions are printed on your proxy card and included in the accompanying Proxy Statement. If you participate virtually in the Annual Meeting, you may vote at that time, even if you previously submitted your vote. Please note, however, that if your shares are held of record by a brokerage firm, bank or other agent and you wish to vote at the Annual Meeting, you must obtain a proxy issued in your name from that agent in order to vote your shares that are held in such agent’s name and account and registered with Computershare as set forth in the proxy statement accompanying this notice.

NOTICE OF 2026 ANNUAL MEETING OF STOCKHOLDERS

By order of the Board of Directors,

Luke J. Albrecht

General Counsel & Secretary

New York, New York

April 30, 2026

TABLE OF CONTENTS

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SYNDAX PHARMACEUTICALS, INC.

730 THIRD AVENUE, FLOOR 9

NEW YORK, NEW YORK 10017

PROXY STATEMENT

FOR THE 2026 ANNUAL MEETING OF STOCKHOLDERS

TO BE HELD ON JUNE 10, 2026

AT 12:00 PM EDT