Adicet Bio, Inc.

ADICET BIO, INC.

131 Dartmouth Street, 3rd Floor

PROXY STATEMENT

FOR THE 2026 ANNUAL MEETING OF STOCKHOLDERS

to be held JUNE 17, 2026

This proxy statement for the 2026 Annual Meeting of Stockholders (Proxy Statement) contains information about the 2026 Annual Meeting of Stockholders (Annual Meeting) of Adicet Bio, Inc., which will be held online on June 17, 2026 at 5:00 p.m. Eastern Time. You may attend the Annual Meeting virtually via the Internet at www.virtualshareholdermeeting.com/ACET2026, where you will be able to vote electronically and submit questions. The board of directors of Adicet Bio, Inc. is using this Proxy Statement to solicit proxies for use at the Annual Meeting. In this Proxy Statement, the terms “Company,” “Adicet Bio,” “Adicet,” “we,” “us,” and “our” refer to Adicet Bio, Inc. The mailing address of our principal executive office is Adicet Bio, Inc., 131 Dartmouth Street, 3rd Floor, Boston, Massachusetts 02116.

All properly submitted proxies will be voted in accordance with the instructions contained in those proxies. If no instructions are specified, the proxies will be voted in accordance with the recommendation of our board of directors with respect to each of the matters set forth in the accompanying Notice of Meeting. You may revoke your proxy at any time before it is exercised at the meeting by giving our corporate secretary written notice to that effect.

This year’s Annual Meeting will be a virtual meeting of the shareholders conducted via live webcast. All shareholders of record on April 21, 2026 are invited to participate in the meeting. We have structured our virtual meeting to provide shareholders the same rights as if the meeting were held in person, including the ability to vote shares electronically during the meeting and ask questions in accordance with the rules of conduct for the meeting. We encourage you to vote your shares prior to the Annual Meeting.

We made this Proxy Statement and our Annual Report on Form 10-K for the fiscal year ended December 31, 2025 available to stockholders on or about April 29, 2026.

Important Notice Regarding the Availability of Proxy Materials for

the Annual Meeting of Stockholders to be Held on June 17, 2026:

This Proxy Statement and our 2025 Annual Report to Stockholders are

available for viewing, printing, and downloading atwww.proxyvote.com.

A copy of our Annual Report on Form 10-K for the fiscal year ended December 31, 2025, as filed with the Securities and Exchange Commission (SEC), except for exhibits, will be furnished without charge to any stockholder upon written request to Adicet Bio, Inc., 131 Dartmouth Street, 3rd Floor, Boston, Massachusetts 02116, Attention: Corporate Secretary. This Proxy Statement and our Annual Report on Form 10-K for the fiscal year ended December 31, 2025 are also available on the SEC’s website atwww.sec.gov.

ADICET BIO, INC

PROXY STATEMENT

FOR THE 2026 ANNUAL MEETING OF STOCKHOLDERS

GENERAL INFORMATION

When are this Proxy Statement and the accompanying materials scheduled to be sent to stockholders?

We have elected to provide access to our proxy materials to our stockholders via the Internet. Accordingly, on or about April 28, 2026, we will begin mailing a Notice of Internet Availability of Proxy Materials (Notice). Our proxy materials, including the Notice of the 2026 Annual Meeting of Stockholders, this Proxy Statement and the accompanying proxy card or, for shares held in street name (i.e., held for your account by a broker or other nominee), a voting instruction form, and the 2025 Annual Report to Stockholders (2025 Annual Report) will be mailed or made available to stockholders on the Internet on or about the same date.

Why did I receive a Notice of Internet Availability of Proxy Materials instead of a full set of proxy materials?

Pursuant to rules adopted by the SEC, for most stockholders, we are providing access to our proxy materials over the Internet rather than printing and mailing our proxy materials. We believe following this process will expedite the receipt of such materials and will help lower our costs and reduce the environmental impact of our Annual Meeting materials. Therefore, the Notice was mailed to holders of record and beneficial owners of our common stock as of April 21, 2026 starting on or about April 29, 2026. The Notice provides instructions as to how stockholders may access and review our proxy materials, including the Notice of the 2026 Annual Meeting of Stockholders, this Proxy Statement, the proxy card and our 2025 Annual Report, on the website referred to in the Notice or, alternatively, how to request that a paper copy of the proxy materials, including a proxy card, be sent to them by mail. The Notice also provides voting instructions. In addition, stockholders of record may request to receive the proxy materials in printed form by mail or electronically by e-mail on an ongoing basis for future stockholder meetings. Please note that, while our proxy materials are available at the website referenced in the Notice, and our Notice of the 2026 Annual Meeting of Stockholders, this Proxy Statement and our 2025 Annual Report are available on our website, no other information contained on either website is incorporated by reference in or considered to be a part of this Proxy Statement.

How do I attend and participate in the Annual Meeting?

To attend and participate in the Annual Meeting, stockholders will need to access the live audio webcast of the meeting. To do so, stockholders of record as of April 21, 2026 will need to visitwww.virtualshareholdermeeting.com/ACET2026 and use their control number found on the proxy card or the Notice, and beneficial owners of shares held in street name will need to follow the same instructions.

You will need the 16-digit control number included on your proxy card or the Notice, as applicable. Instructions on how to connect to the Annual Meeting and participate via the Internet, including how to demonstrate proof of stock ownership, are posted atwww.virtualshareholdermeeting.com/ACET2026. If you do not have your 16-digit control number, you will be able to access and listen to the Annual Meeting but you will not be able to vote your shares or submit questions during the Annual Meeting.

The live audio webcast of the Annual Meeting will begin promptly at 5:00 p.m. Eastern Time. We encourage stockholders to login to this website and access the webcast before the Annual Meeting’s start time. Online check-in will begin, and stockholders may begin submitting written questions, at 4:45 p.m. Eastern Time, and you should allow ample time for the check-in procedures.

How can I get help if I have trouble checking in or listening to the meeting online?

We will have technicians ready to assist you with any technical difficulties you may have accessing the virtual meeting or submitting questions. If you encounter any difficulties accessing the virtual meeting during the check-in or meeting time, please call the technical support number that will be posted on the Virtual Shareholder Meeting log in page.

Who is soliciting my vote?

Our board of directors is soliciting your vote for the Annual Meeting.

When is the record date for the Annual Meeting?

The record date for determination of stockholders entitled to vote at the Annual Meeting is the close of business on April 21, 2026.

How many votes can be cast by all stockholders?

There were 9,598,201 shares of our common stock, par value $0.0001 per share, outstanding on April 21, 2026, all of which are entitled to vote with respect to all matters to be acted upon at the Annual Meeting. Each stockholder of record is entitled to one vote for each share of our common stock held by such stockholder. There are no cumulative voting rights. None of our shares of undesignated preferred stock were outstanding as of April 21, 2026.

Who is entitled to vote?

Holders of record of our common stock at the close of business on April 21, 2026, the record date for the Annual Meeting, are entitled to receive notice of the Annual Meeting and to vote at the Annual Meeting. If you are a holder of record of our common stock as of the record date, you may vote the shares that you held on the record date even if you sell such shares after the record date. Each outstanding share as of the record date entitles its holder to cast one vote for each matter to be voted upon and, with respect to the election of directors, one vote for each director to be elected.

Registered Stockholders. If shares of our common stock are registered directly in your name with our transfer agent, you are considered the stockholder of record with respect to those shares. As the stockholder of record, you have the right to grant your voting proxy directly to the individuals listed on the proxy card or vote on your own behalf at our virtual Annual Meeting. Throughout this Proxy Statement, we refer to these registered stockholders as "stockholders of record."

Street Name Stockholders. If shares of our common stock are held on your behalf in a brokerage account or by a bank or other nominee, you are considered to be the beneficial owner of shares that are held in "street name," and the proxy materials were forwarded to you by your broker or nominee, who is considered the stockholder of record with respect to those shares. As the beneficial owner, you have the right to direct your broker, bank or other nominee as to how to vote your shares. Beneficial owners are also invited to attend our virtual Annual Meeting. However, since a beneficial owner is not the stockholder of record, you may not vote your shares of our common stock on your own behalf at the Annual Meeting unless you follow your broker's procedures for obtaining a legal proxy. Note you should also be receiving a voting instruction form for you to use from your broker. Throughout this Proxy Statement, we refer to stockholders who hold their shares through a broker, bank or other nominee as "street name stockholders."

How do I vote?

If you are a stockholder of record, there are several ways for you to vote your shares.

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By Internet. You may vote atwww.proxyvote.com, 24 hours a day, seven days a week, by following the instructions at that site for submitting your proxy electronically or scanning the QR code with your smartphone. You will be required to enter the 16-digit control number provided on your proxy card or voting instruction form. Votes submitted through the Internet prior to the commencement of the Annual Meeting.

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By Telephone. You may vote using a touch-tone telephone by calling 1-800-690-6903, 24 hours a day, seven days a week. You will be required to enter the 16-digit control number provided on your proxy card or voting instruction form. Votes submitted by telephone prior to the commencement of the Annual Meeting.

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By Mail. If you requested and received a printed copy of the proxy materials, you may vote by mail by completing, signing, and dating the enclosed proxy card and returning it in the enclosed prepaid envelope. Votes submitted through the mail must be received prior to June 16, 2026.

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During the Annual Meeting. If you attend the Annual Meeting online, you may vote your shares online while virtually attending the Annual Meeting by visitingwww.virtualshareholdermeeting.com/ACET2026. You will need your control number provided on your proxy card or the Notice in order to be able to vote during the Annual Meeting.

If you do not wish to vote in person or will not be attending the Annual Meeting, you may vote by proxy. You can vote by proxy over the Internet by following the instructions provided in the Notice, or, if you requested printed copies of the proxy materials by mail, you can vote by mailing your proxy as described in the proxy materials. In order to be counted, proxies submitted by Internet must be received by the cutoff time of 11:59 p.m. Eastern Time on June 16, 2026. Proxies submitted by mail must be received before the start of the Annual Meeting. If you complete and submit your proxy before the Annual Meeting, the persons named as proxies will vote the shares represented by your proxy in accordance with your instructions. If you submit a proxy without giving voting instructions, your shares will be voted in the manner recommended by our board of directors on all matters presented in this Proxy Statement, and as the persons named as proxies may determine in their discretion with respect to any other matters properly presented at the Annual Meeting. You may also authorize another person or persons to act for you as proxy in a writing, signed by you or your authorized representative, specifying the details of those proxies’ authority. The original writing must be given to each of the named proxies, although it may be sent to them by electronic transmission if, from that transmission, it can be determined that the transmission was authorized by you.

Even if you plan to participate in our virtual Annual Meeting, we recommend that you also vote by proxy so that your vote will be counted if you later decide not to participate in the Annual Meeting. If you submit a proxy via the Internet, by telephone, or by mail, your voting instructions authorize the proxy holders in the same manner as if you signed, dated, and returned your proxy card. If you submit a proxy via the Internet, by telephone, or by mail, you do not need to return your proxy card.

If any other matters are properly presented for consideration at the Annual Meeting, including, among other things, consideration of a motion to adjourn the Annual Meeting to another time or place (including, without limitation, for the purpose of soliciting additional proxies), the persons named in your proxy and acting thereunder will have discretion to vote on those matters in accordance with their best judgment. We do not currently anticipate that any other matters will be raised at the Annual Meeting.

How do I revoke my proxy?

You may revoke your proxy by (1) following the instructions on the Notice and entering a new vote by mail that we receive before the start of the Annual Meeting or over the Internet by the cutoff time of 11:59 p.m. Eastern Time on June 16, 2026, (2) attending and voting at the Annual Meeting (although attendance at the Annual Meeting will not in and of itself revoke a proxy), or (3) by filing an instrument in writing revoking the proxy or another duly executed proxy bearing a later date with our Corporate Secretary. Any written notice of revocation or subsequent proxy card must be received by our Corporate Secretary prior to the taking of the vote at the Annual Meeting. Such written notice of revocation or subsequent proxy card should be hand delivered to our Corporate Secretary or sent to our principal executive offices at Adicet Bio, Inc., 131 Dartmouth Street, 3 rd Floor, Boston, Massachusetts 02116, Attention: Corporate Secretary.

If a broker, bank, or other nominee holds your shares, you must contact such broker, bank, or nominee in order to find out how to change your vote.

How is a quorum reached?

Our Amended and Restated Bylaws (bylaws) provide that a majority of the shares entitled to vote, present in person or represented by proxy, will constitute a quorum for the transaction of business at the Annual Meeting.

Under the General Corporation Law of the State of Delaware, shares that are voted “abstain” or “withheld” and broker “non-votes” are counted as present for purposes of determining whether a quorum is present at the Annual Meeting. If a quorum is not present, the meeting may be adjourned until a quorum is obtained. There were 9,598,201 shares of common stock outstanding and entitled to vote on April 21, 2026, our record date. Therefore, a quorum will be present if 4,799,101 shares of our common stock are present in person or represented by executed proxies timely received by us at the Annual

Meeting. Shares present virtually during the Annual Meeting will be considered shares of common stock represented in person at the meeting.

How is the vote counted?

Under our bylaws, directors are elected by a plurality of the votes properly cast. This means that the two director nominees receiving the highest number of affirmative votes will be elected as directors. You may vote for all the director nominees, withhold authority to vote your shares for all the director nominees or withhold authority to vote your shares with respect to any one or more of the director nominees. Withholding authority to vote your shares with respect to one or more director nominees will have no effect on the election of those nominees. Broker “non-votes” (as defined below) will have no effect on the election of the nominees.

Under our bylaws, any proposal other than an election of directors is decided by a majority of the votes properly cast for and against such proposal, except where a larger vote is required by law or by our Third Amended and Restated Certificate of Incorporation, or certificate of incorporation, or bylaws. Abstentions and broker “non-votes” do not have an impact on such proposals. Proposal Nos. 2 and 3 require an affirmative vote of a majority of votes properly cast for and against such proposal, and abstention and broker “non-votes,” if any, will have no effect on these proposals.

If you authorize your proxy to vote your shares electronically via the Internet or by telephone, or, if you properly marked, signed, dated and returned the proxy card mailed to you, the shares that the proxy represents will be voted in the manner specified on the proxy. If you properly signed and returned a proxy card but no specification is made, your shares will be voted (i) “for all” on the election of the director nominees named in this Proxy Statement, (ii) “for” the ratification of a non-binding advisory vote on compensation of our named executive officers, and (iii) “for” ratification of the appointment of KPMG LLP as our independent registered public accounting firm for the fiscal year ending December 31, 2026.

If your shares are held in “street name” by a brokerage firm, your brokerage firm is required to vote your shares according to your instructions. Pursuant to the New York Stock Exchange (NYSE) rules, which are applicable to NYSE-member brokerage firms, if you do not give instructions to your brokerage firm, the brokerage firm will still be able to vote your shares with respect to “discretionary” items, but will not be allowed to vote your shares with respect to “non-discretionary” items. The ratification of the appointment of KPMG LLP as our independent registered public accounting firm (Proposal No. 3) is considered to be a discretionary item under the NYSE rules and your broker or nominee will be able to vote on that item even if it does not receive instructions from you. The election of directors (Proposal No. 1) and the ratification of, on a non-binding advisory basis, the compensation of our named executive officers (Proposal No. 2) are considered non-discretionary items. A broker or nominee may not vote your shares with respect to these non-discretionary items if you have not provided instructions. This is called a “broker non-vote.” We strongly encourage you to submit your proxy with instructions and exercise your right to vote as a stockholder.

Who pays the cost for soliciting proxies?

We are making this solicitation and will pay the entire cost of preparing and distributing the Notice and our proxy materials and soliciting votes. If you choose to access the proxy materials or vote over the Internet, you are responsible for any Internet access charges that you may incur. Our officers and employees may, without compensation other than their regular compensation, solicit proxies through further mailings, personal conversations, facsimile transmissions, e-mails, or otherwise. We have hired Broadridge Financial Solutions, Inc. to assist us in the distribution of proxy materials described above. Proxy solicitation expenses that we will pay include those for preparation, mailing, returning, and tabulating the proxies.

How may stockholders submit matters for consideration at an annual meeting?

Under our bylaws, certain procedures are provided that a stockholder must follow to nominate persons for election as directors or to introduce an item of business at an annual meeting of stockholders. These procedures provide that nominations for director nominees and/or an item of business to be introduced at an annual meeting of stockholders must be timely submitted in writing to the Secretary of the Company, at 131 Dartmouth Street, 3 rd Floor Boston, Massachusetts 02116 or by email at IR@adicetbio.com. To be considered timely, the required notice must be in writing and received by our corporate secretary at our principal executive offices not less than 90 days nor more than 120 days prior to the first anniversary of the preceding year’s annual meeting. However, in the event that the date of the annual meeting is advanced by more than 30 days, or delayed by more than 60 days, from the first anniversary of the preceding year’s annual meeting, or if no annual meeting were held in the preceding year, a stockholder’s notice must be so received no earlier than the 120 th day prior to such annual meeting and not later than the close of business on the later of (A) the 90 th day prior to such annual meeting and (B) the tenth day following the day on which notice of the date of such annual meeting was mailed or public disclosure of the date of such annual meeting was made, whichever first occurs.

In addition, any stockholder who, in accordance with Rule 14a-8 of the Securities Exchange Act of 1934, as amended (Exchange Act), wishes to present a proposal intended to be included in the proxy statement for the next annual meeting of our stockholders in 2027 must submit such proposals to us not later than December 29, 2026. These proposals must comply with all rules and regulations of the SEC, including Rule 14a-8 under the Exchange Act. Additionally, stockholders who intend to solicit proxies in support of director nominees other than the Company’s nominees must provide notice that sets forth the information required by Rule 14a-19 under the Exchange Act by the same deadline noted herein to submit a notice of nomination for consideration at next year’s annual meeting. If the date of the annual meeting is moved by more than 30 days from the date contemplated at the time of the previous year’s proxy statement, then notice must be received within a reasonable time before we begin to print and send proxy materials. If that happens, we will publicly announce the deadline for submitting a proposal in a press release or in a document filed with the SEC.

How can I know the voting results?

We plan to announce preliminary voting results at the Annual Meeting and will publish final results in a Current Report on Form 8-K to be filed with the SEC within four business days following the Annual Meeting. If final voting results are not available to us in time to file a Form 8-K within four business days after the Annual Meeting, we intend to file a Form 8-K to publish preliminary results and, within four business days after the final results are known to us, file an additional Form 8-K to publish the final results.