Sensei Biotherapeutics, 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

Filed by the Registrant ☒

Filed by a Party other than the Registrant ☐

Check the appropriate box:

Sensei Biotherapeutics, 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):

Dear Sensei Stockholders:

You are cordially invited to attend the annual meeting of stockholders of Sensei Biotherapeutics, Inc. (“Sensei,” the “Company,” “we,” “our” or “us”), which will be held on Wednesday, June 10, 2026 at 11:00 a.m. Eastern Daylight Time (the “Annual Meeting”), unless postponed or adjourned to a later date. To facilitate stockholder participation in the Annual Meeting, the Annual Meeting will be held through a live webcast at www.proxydocs.com/SNSE. You will not be able to attend the meeting in person. The record date for the Annual Meeting is April 13, 2026. Only stockholders of record at the close of business on that date are entitled to notice of and to vote at the Annual Meeting or any adjournment thereof.

On February 17, 2026, the Company acquired Faeth Holdings Therapeutics, Inc., a Delaware corporation (“Faeth HoldCo”) and Faeth Therapeutics, LLC, a Delaware limited liability company and wholly owned subsidiary of Faeth HoldCo (“Faeth Subsidiary” and, together with Faeth HoldCo, “Faeth” or “Faeth Therapeutics”), in accordance with the terms of the Agreement and Plan of Merger, dated February 17, 2026 (the “Merger Agreement”), by and among the Company, Sapphire First Merger Sub, Inc., a Delaware corporation and a wholly owned subsidiary of the Company (“First Merger Sub”), Sapphire Second Merger Sub, LLC, a Delaware limited liability company and wholly owned subsidiary of the Company (“Second Merger Sub”), Faeth HoldCo and Faeth Subsidiary. Pursuant to the Merger Agreement, First Merger Sub merged with and into Faeth HoldCo, pursuant to which Faeth HoldCo was the surviving corporation and became a wholly owned subsidiary of the Company (the “First Merger”). Immediately following the First Merger, Faeth HoldCo merged with and into Second Merger Sub, pursuant to which Second Merger Sub was the surviving entity (together with the First Merger, the “Acquisition”).

Under the terms of the Merger Agreement, following the closing of the Acquisition (the “Closing”), (i) the Company issued to the stockholders of Faeth HoldCo an aggregate of 10,497.0980 shares of Series B Non-Voting Convertible Preferred Stock, par value $0.0001 per share (the “Series B Preferred Stock”) (as described below), each share of which is convertible into 1,000 shares of common stock of the Company, par value $0.0001 per share (“common stock”) (representing 10,497,098 shares on anas-converted-to-common basis and without giving effect to any beneficial ownership limitations), subject to certain conditions described below, (ii) an outstanding warrant to purchase shares of Faeth Subsidiary capital stock was converted into a warrant to purchase an aggregate of 2.1020 shares of Series B Preferred Stock (representing a warrant to purchase 2,102 shares of common stock on anas-converted-to-common basis and without giving effect to any beneficial ownership limitations) and (iii) all outstanding options to purchase Faeth Subsidiary common stock were assumed by the Company and converted into options to purchase an aggregate of 252,210 shares of common stock.

In connection with the Acquisition, on February 17, 2026, the Company entered into a Securities Purchase Agreement (the “Purchase Agreement”) with the purchasers named therein (the “Investors”), pursuant to which, on February 20, 2026 (the “PIPE Closing”), the Company issued and sold an aggregate of 14,440.395 shares of Series B Preferred Stock (the “PIPE Securities”) for an aggregate cash purchase price of approximately $200 million (such transaction, the “2026 Private Placement” and, together with the Acquisition and the other transactions and actions contemplated by the Merger Agreement, the “Acquisition Transactions”).

Subject to the receipt of stockholder approval of Proposals No. 3 and No. 4 (the “Required Company Stockholder Matters”), each outstanding share of Series B Preferred Stock will automatically convert into 1,000 shares of common stock, subject to certain beneficial ownership limitations established by each holder. As a result of the transactions, immediately following the PIPE Closing, equityholders of the Company immediately prior to the acquisition owned approximately 4.9% of the common stock, equityholders of Faeth Therapeutics immediately prior to the acquisition owned approximately 40.6% of the common stock and the Investors in the 2026 Private Placement owned approximately 54.5% of the common stock, in each case, calculated on a fully-diluted, as-converted-to-common basis (and without giving effect to any beneficial ownership limitations) using the treasury stock method and based on the implied equity values of the Company and Faeth Therapeutics.

At the Annual Meeting, we will ask our stockholders:

As described in the accompanying proxy statement, in connection with the execution of the Merger Agreement, the Company and Faeth Subsidiary entered into stockholder support agreements (the “Support Agreements”) with certain of the Company’s officers and directors (solely in their capacity as stockholders), representing approximately 1.5% of shares of common stock outstanding on the record date. The Support Agreements provide that, among other things, each of the parties thereto has agreed to vote or cause to be voted all of the shares of common stock owned by such stockholder in favor of Proposals Nos. 3, 4, 5 and 6 at the Annual Meeting, subject to and in accordance with the terms of the Support Agreements.

After careful consideration, the Board has unanimously determined to recommend that our stockholders vote “FOR” each nominee named in the Director Election Proposal, “FOR” the Auditor Ratification Proposal, “FOR” the Conversion Proposal, “FOR” the Authorized Share Proposal, “FOR” the 2026 Plan Proposal, “FOR” the 2026 ESPP Proposal and “FOR” the Adjournment Proposal.

Shares of our common stock are currently listed on The Nasdaq Capital Market under the symbol “SNSE.” If the Required Company Stockholder Matters are approved and our Nasdaq Listing Application is approved, it is expected that we will change our name to “Faeth Therapeutics, Inc.” and our common stock will trade on The Nasdaq Capital Market under the symbol “FTH.”

More information about the Acquisition and the Required Company Stockholder Matters is contained in the accompanying proxy statement. We urge you to read the proxy statement carefully and in its entirety.

Your vote is important. Whether or not you expect to attend the Annual Meeting, please submit voting instructions for your shares promptly by using the directions on your Notice, or, if you elected to receive printed proxy materials by mail, on your proxy card, to vote by one of the following methods: (1) over the Internet before the Annual Meeting at www.proxypush.com/SNSE and during the Annual Meeting at www.proxydocs.com/SNSE, (2) by telephone by calling the toll-free number (866)284-5317, or (3) if you elected to receive printed proxy materials by mail, by marking, dating, and signing your proxy card and returning it in the accompanying postage-paid envelope. Even if you have voted by proxy, you may still vote online if you attend the virtual Annual Meeting. Please note, however, that if your shares are held of record by a broker, bank, or other nominee and you wish to vote online at the Annual Meeting, you must obtain a proxy issued in your name from that record holder.

We thank you for your consideration and continued support.

Yours sincerely,

/s/ Christopher W. Gerry

Christopher W. Gerry President, General Counsel and Secretary Rockville, Maryland

This proxy statement is dated April 27, 2026 and is first being mailed to stockholders on or about April 27, 2026.

1405 Research Boulevard, Suite 125

Rockville, Maryland 20850

NOTICE OF ANNUAL MEETING OF STOCKHOLDERS

To Be Held On June 10, 2026

Dear Stockholder:

The Annual Meeting of stockholders of Sensei Biotherapeutics, Inc. (the “Company”) will be held on Wednesday, June 10, 2026 at 11:00 a.m. Eastern Daylight Time (the “Annual Meeting”). To facilitate stockholder participation in the Annual Meeting, the Annual Meeting will be held through a live webcast at www.proxydocs.com/SNSE. You will not be able to attend the meeting in person. The meeting will be held for the following purposes:

These items of business are more fully described in the Proxy Statement accompanying this Notice. This year’s Annual Meeting will be held virtually through a live webcast. You will be able to attend the Annual Meeting, submit questions and vote during the live webcast by visiting www.proxydocs.com/SNSE and entering the Control Number included in your proxy card, voting instruction form, or in the instructions that you received via email. Please refer to the additional logistical details and recommendations in the accompanying proxy statement. You maylog-in beginning at 10:45 a.m. Eastern Daylight Time, on Wednesday, June 10, 2026. The record date for the Annual Meeting is April 13, 2026. Only stockholders of record at the close of business on that date are entitled to notice of and to vote at the meeting or any adjournment thereof.

By Order of the Board of Directors,

Christopher W. Gerry

President, General Counsel and Secretary

Rockville, Maryland

April 27, 2026

We are primarily providing access to our proxy materials over the internet pursuant to the Securities and Exchange Commission’s notice and access rules. On or about April 27, 2026, we expect to mail to our stockholders a Notice of Internet Availability of Proxy Materials that will indicate how to access our 2026 Proxy Statement and 2025 Annual Report on the internet and will include instructions on how you can receive a paper copy of the annual meeting materials, including the notice of annual meeting, proxy statement and proxy card.

Whether or not you expect to attend the virtual Annual Meeting, please submit voting instructions for your shares promptly using the directions on your Notice, or, if you elected to receive printed proxy materials by mail, your proxy card, to vote by one of the following methods: (1) over the Internet before the Annual Meeting at www.proxypush.com/SNSE and during the Annual Meeting at www.proxydocs.com/SNSE, (2) by telephone by calling the toll-free number (866)284-5317, or (3) if you elected to receive printed proxy materials by mail, by marking, dating, and signing your proxy card and returning it in the accompanying postage-paid envelope. Even if you have voted by proxy, you may still vote online if you attend the virtual Annual Meeting. Please note, however, that if your shares are held of record by a broker, bank, or other nominee and you wish to vote online at the Annual Meeting, you must obtain a proxy issued in your name from that record holder.