Cartesian Therapeutics, Inc.

This proxy statement is furnished in connection with the solicitation by the Board of Directors (the “Board”) of Cartesian Therapeutics, Inc. (the “Company”) of proxies to be voted at our Annual Meeting of Stockholders to be held on Friday, June 14, 2024 (the “Annual Meeting”) at 10:00 a.m., Eastern Time, and at any continuation, postponement, or adjournment of the Annual Meeting. The Annual Meeting will be a completely virtual meeting, which will be conducted via live webcast. You will be able to attend the Annual Meeting online and submit your questions during the meeting by visitingwww.virtualshareholdermeeting.com/RNAC2024 and entering your 16-digit control number included on your Notice of Internet Availability of Proxy Materials (the “Internet Notice”), on your proxy card or on the instructions that accompanied your proxy materials. Holders of record of shares of common stock, $0.0001 par value (“Common Stock”), as of the close of business on April 18, 2024 (the “Record Date”), will be entitled to notice of and to vote at the Annual Meeting and any continuation, postponement, or adjournment of the Annual Meeting. As of the Record Date, there were 17,796,053 shares of Common Stock outstanding and entitled to vote at the Annual Meeting. Each share of Common Stock is entitled to one vote on any matter presented to stockholders at the Annual Meeting.

This proxy statement and the Company’s Annual Report on Form 10-K for the year ended December 31, 2023, (the “2023 Annual Report”) will be released on or about April 26, 2024 to our stockholders on the Record Date.

In this proxy statement, “Cartesian”, “Company”, “we”, “us”, and “our” refer to Cartesian Therapeutics, Inc.

IMPORTANT NOTICE REGARDING THE AVAILABILITY OF PROXY MATERIALS

FOR THE STOCKHOLDER MEETING TO BE HELD ON FRIDAY, JUNE 14, 2024

This Proxy Statement and the 2023 Annual Report are available at http://www.proxyvote.com

PROPOSALS

At the Annual Meeting, our stockholders will be asked:

1. To elect Carrie S. Cox and Murat Kalayoglu, M.D., Ph.D. as Class II Directors to serve until the 2027 Annual Meeting of Stockholders, and until their respective successors shall have been duly elected and qualified;

2. To approve, on a non-binding and advisory basis, a resolution approving the compensation of our named executive officers, as described in the accompanying proxy statement under “Executive and Director Compensation”;

3. To approve the Cartesian Therapeutics, Inc. Amended and Restated 2016 Incentive Award Plan, which is an amendment and restatement of the Selecta Biosciences, Inc. 2016 Incentive Award Plan;

4. To ratify the appointment of Ernst & Young LLP as our independent registered public accounting firm for the fiscal year ending December 31, 2024; and

5. To transact such other business as may properly come before the Annual Meeting or any continuation, postponement, or adjournment of the Annual Meeting.

Stockholders at an annual meeting will only be able to consider proposals or nominations specified in the Notice of Annual Meeting or brought before the meeting by or at the direction of our Board or by a stockholder of record on the Record Date for the meeting who is entitled to vote at the meeting and who has delivered timely written notice in proper form to our Secretary of the stockholder’s intention to bring such business before the meeting. As of the date of this proxy statement, we know of no other business that will be presented at the Annual Meeting. If any other matter properly comes before the stockholders for a vote at the Annual Meeting, however, the proxy holders named on the Company’s proxy card will vote your shares in accordance with their best judgment.

RECOMMENDATIONS OF THE BOARD

The Board recommends that you vote your shares as indicated below. If you return a properly completed proxy card, or vote your shares by telephone or Internet, your shares of Common Stock will be voted on your behalf as you direct. If not otherwise specified, the shares of Common Stock represented by the proxies will be voted in accordance with the recommendations of the Board. The Board recommends that you vote:

1. FOR the election of Carrie S. Cox and Murat Kalayoglu, M.D., Ph.D. as Class II Directors;

2. FOR the approval, on a non-binding, advisory basis, of the compensation of our named executive officers;

3. FOR the approval of the Cartesian Therapeutics, Inc. Amended and Restated 2016 Incentive Award Plan;

4. FOR the ratification of the appointment of Ernst & Young LLP as our independent registered public accounting firm for the fiscal year ending December 31, 2024.

If any other matter properly comes before the stockholders for a vote at the Annual Meeting, the proxy holders named on our proxy card will vote your shares in accordance with their best judgment.

INFORMATION ABOUT THIS PROXY STATEMENT

Why you received this Proxy Statement. You are viewing or have received these proxy materials because Cartesian’s Board is soliciting your proxy to vote your shares at the Annual Meeting. This proxy statement includes information that we are required to provide to you under the rules of the Securities and Exchange Commission (“SEC”) and that is designed to assist you in voting your shares.

Notice of Internet Availability of Proxy Materials. As permitted by SEC rules, Cartesian is making this proxy statement and the 2023 Annual Report available to its stockholders electronically via the Internet. On or about April 26, 2024, we mailed to our stockholders an Internet Notice containing instructions on how to access this proxy statement and our 2023 Annual Report and vote online. If you received an Internet Notice by mail, you will not receive a printed copy of the proxy materials in the mail unless you specifically request them. Instead, the Internet Notice instructs you on how to access and review all of the important information contained in the proxy statement and 2023 Annual Report. The Internet Notice also instructs you on how you may submit your proxy over the Internet. If you received an Internet Notice by mail and would like to receive a printed copy of our proxy materials, you should follow the instructions for requesting such materials contained in the Internet Notice.

Printed Copies of Our Proxy Materials. If you received printed copies of our proxy materials, then instructions regarding how you can vote are contained on the proxy card included in the materials.

Householding. The SEC’s rules permit us to deliver a single set of proxy materials to one address shared by two or more of our stockholders. This delivery method is referred to as “householding” and can result in significant cost savings. To take advantage of this opportunity, we have delivered only one set of proxy materials to multiple stockholders who share an address, unless we received contrary instructions from the impacted stockholders prior to the mailing date. We agree to deliver promptly, upon written or oral request, a separate copy of the proxy materials, as requested, to any stockholder at the shared address to which a single copy of those documents was delivered. If you prefer to receive separate copies of the proxy materials, contact Broadridge Financial Solutions, Inc. at (866) 540-7095 or in writing at Broadridge, Householding Department, 51 Mercedes Way, Edgewood, New York 11717.

If you are currently a stockholder sharing an address with another stockholder and received more than one copy of proxy materials, but wish to receive only one copy of future proxy materials for your household, please contact Broadridge at the above phone number or address.