Cullinan Therapeutics, Inc.

FOR THE 2026 ANNUAL MEETING OF STOCKHOLDERS

to be held June 16, 2026

This proxy statement contains information about the 2026 Annual Meeting of Stockholders (the “Annual Meeting”) of Cullinan Therapeutics, Inc., which will be held on June 16, 2026 at 10:00 a.m. Eastern Time. The Annual Meeting will be a virtual meeting in order to facilitate stockholder attendance and participation by enabling stockholders to participate from any location and at no cost to the stockholder. The virtual meeting will be conducted via live webcast. You will be able to virtually attend the Annual Meeting as well as vote and submit your questions during the live webcast of the meeting by visiting www.virtualshareholdermeeting.com/CGEM2026 and entering the 16-digit control number included in our Notice of Internet Availability of the proxy materials, on your proxy card or in the instructions that accompanied your proxy materials. In this proxy statement, the terms “Cullinan Therapeutics,” “Cullinan,” “we,” “us,” and “our” refer to Cullinan Therapeutics, Inc. The mailing address of our principal executive office is Cullinan Therapeutics, Inc., One Main Street, Suite 1350, Cambridge, Massachusetts 02142.

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 Annual Meeting by giving our Corporate Secretary written notice to that effect.

We made this proxy statement and our Annual Report to Stockholders for the fiscal year ended December 31, 2025 (the “2025 Annual Report”) available to stockholders on or about April 28, 2026.

We are a “smaller reporting company” as defined in Rule 12b-2 under the Securities Exchange Act of 1934, as amended. The SEC has adopted rules allowing smaller reporting companies to provide scaled disclosures, and we are permitted to rely on these exemptions from certain disclosure requirements for as long as we remain a smaller reporting company. We may take advantage of scaled disclosures available to smaller reporting companies until the fiscal year following the determination that either (i) the market value of our voting and non-voting common stock held by non-affiliates is greater than $700 million or (ii) the market value of our voting and non-voting common stock held by non-affiliates is less than $700 million but greater than $250 million and our annual revenues are greater than $100 million, each determined on an annual basis. Accordingly, our executive compensation disclosure, pay versus performance analysis, related party transaction reporting, and other disclosures in this proxy statement comply with the requirements for smaller reporting companies where applicable.

Important Notice Regarding the Availability of Proxy Materials for

the Annual Meeting of Stockholders to be Held on June 16,2026:

This proxy statement and our 2025 Annual Report are

available for viewing, printing and downloading at www.proxyvote.com.

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 the Notice of Internet Availability (“Notice of Availability”). 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, bank or other nominee), a voting instruction form, and the 2025 Annual Report will be 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 proxy materials. Therefore, the Notice of Availability was mailed to holders of record and beneficial owners of our common stock starting on or about April 28, 2026. The Notice of Availability 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 of Availability or, alternatively, how to request that a printed copy of the proxy materials, including a proxy card, be sent to them by mail. The Notice of Availability 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 of Availability and our Notice of the 2026 Annual Meeting of Stockholders, and 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.

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 20, 2026.

How many votes can be cast by all stockholders?

There were 61,451,380 shares of our common stock, par value $0.0001 per share, outstanding on April 20, 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. During the Annual Meeting, the list of stockholders will be available for examination at www.virtualshareholdermeeting.com/CGEM2026.

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 at www.proxyvote.com 24 hours a day, seven days a week, by following the instructions at that site for submitting your proxy electronically. You will be required to enter the control number provided in the Notice of Availability or the proxy card. Votes submitted through the Internet must be received by 11:59 p.m. Eastern Time, June 15, 2026.

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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 provide the control number provided in the Notice of Availability or the proxy card. Votes submitted by telephone must be received by 11:59 p.m. Eastern Time, June 15, 2026.

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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 by June 15, 2026.

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During the Annual Meeting. To vote during the live webcast of the Annual Meeting, visit www.virtualshareholdermeeting.com/CGEM2026. You will be required to enter the 16-digit control number provided in the Notice of Availability or the proxy card.

If the Annual Meeting is adjourned or postponed, the deadlines above may be extended.

If you are a beneficial owner of shares held in “street name” by your broker, bank or other nominee, you should have received a voting instruction form with these proxy materials from your broker, bank or other nominee rather than from us. The voting deadlines and availability of telephone and Internet voting for beneficial owners of shares will depend on the voting processes of the broker, bank or other nominee that holds your shares. Therefore, we urge you to carefully review and follow the voting instruction form and any other materials that you receive from that organization. If you hold your shares of Cullinan Therapeutics common stock in multiple accounts, you should vote your shares as described in each set of proxy materials you receive.

If you submit a proxy without giving voting instructions, your shares will be voted in the manner recommended by the board of directors on all matters presented in this proxy statement, and as the persons named as proxies in the proxy card 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.

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 can I virtually attend the Annual Meeting?

To virtually attend and participate in the Annual Meeting, stockholders will need to access the live webcast of the meeting. To do so, stockholders of record will need to visit www.virtualshareholdermeeting.com/CGEM2026 and enter the 16-digit control number provided in the Notice of Availability and beneficial owners of shares held in street name will need to follow the instructions provided in the voting instructions form by the broker, bank or other nominee that holds their shares.

The live webcast of the Annual Meeting will begin promptly at 10:00 a.m. Eastern Time on June 16, 2026. We encourage stockholders to login to the website and access the webcast before the Annual Meeting’s start time.

Additionally, questions regarding how to virtually attend and participate via the Internet can be answered by following the assistance instructions included at www.virtualshareholdermeeting.com/CGEM2026 or by calling the phone number (844) 986-0822 if you are located in the United States or (303) 562-9302 if you are located internationally.

If you wish to submit a question during the Annual Meeting, follow the onscreen instructions. Our Annual Meeting will be governed by the Annual Meeting’s Rules of Conduct which will be posted on www.virtualshareholdermeeting.com/CGEM2026 on the day of the Annual Meeting.

How do I revoke my proxy?

If you are a stockholder of record, you may revoke your proxy by (1) following the instructions on the Notice of Availability and submitting a new vote by Internet, telephone or mail using the procedures described in the “How do I vote?” section above before the applicable deadline, (2) virtually attending and voting during 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 Cullinan Therapeutics, Inc., One Main Street, Suite 1350, Cambridge, Massachusetts 02142, 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 third amended and restated bylaws (“bylaws”) provide that a majority of the shares entitled to vote, present virtually 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.

What vote is required to approve each matter and how is the vote counted?

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 second amended and restated certificate of incorporation, as amended (“certificate of incorporation”) or by our bylaws. Abstentions and “broker non-votes” are not included in the tabulation of the voting results on any such proposal and, therefore, do not have an impact on such proposals. A “broker non-vote” occurs when a nominee holding shares for a beneficial owner does not vote on a particular proposal because the nominee does not have discretionary voting power with respect to that item and has not received instructions from the beneficial owner.

If your shares are held in “street name” by a broker, bank or other nominee, your broker, bank or other nominee is required to vote your shares according to your instructions. If you do not give instructions to your broker, bank or other nominee, the broker, bank or other nominee will still be able to vote your shares with respect to certain “discretionary” items but will not be allowed to vote your shares with respect to “non-discretionary” items.

The election of directors (Proposal No. 1) is considered to be a “non-discretionary” item. If you do not instruct your broker how to vote with respect to this proposal, your broker, bank or other nominee may not vote for this proposal, and those votes will be counted as “broker non-votes.” To be elected, the directors nominated via Proposal No. 1 must receive a plurality of the votes cast and entitled to vote on the proposal, meaning that the director nominees receiving the most FOR votes will be elected. Shares voting “withheld” and “broker non-votes” will have no effect on the election of directors.

The ratification of the appointment of KPMG LLP as our independent registered public accounting firm for the 2026 fiscal year (Proposal No. 2) is considered to be a “discretionary” item, and your broker, bank or other nominee will be able to vote on this proposal even if it does not receive instructions from you. Thus, we do not expect any “broker non-votes” on Proposal No. 2.

Approval of the advisory vote on named executive officer compensation (Proposal No. 3) is considered to be a “non-discretionary” item. If you do not instruct your broker how to vote with respect to this proposal, your broker, bank or other nominee may not vote for this proposal, and those votes will be counted as “broker non-votes.”

Who pays the cost for soliciting proxies?

We will pay the entire cost of preparing and distributing the Notice of Availability and our proxy materials and soliciting votes. 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.

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.