Filed Pursuant to Rule 424(b)(3)
Registration No. 333-279452

PROSPECTUS

Cullinan Therapeutics, Inc.

Up to 14,421,070 Shares of Common Stock

Pre-Funded Warrants to Purchase Up to 315,790 Shares of Common Stock

This prospectus relates to the disposition, from time to time, by the selling stockholders (including their transferees, pledgees, donees or successors) identified in this prospectus under the caption “Selling Stockholders” on page 12 of (i) up to 14,421,070 shares of our common stock, par value $0.0001 per share (the “Common Stock”) and (ii) up to 315,790 shares of Common Stock issuable upon the exercise of pre-funded warrants, with an exercise price of $0.001 per share (the “Pre-Funded Warrants”), that we issued in a private placement completed on April 18, 2024 (the “Private Placement”).

The selling stockholders may, but are not required to, sell, transfer, or otherwise dispose of any or all of the shares of Common Stock offered by this prospectus from time to time in a number of different ways and at varying prices as determined by the prevailing market price for shares or in negotiated transactions. See “Plan of Distribution” on page 17 for a description of how the selling stockholders may dispose of the shares covered by this prospectus. We do not know when or in what amount the selling stockholders may offer the shares for sale.

We are not selling any shares of Common Stock under this prospectus and will not receive any proceeds from the sale of shares of Common Stock by the selling stockholders, except with respect to amounts received by us upon exercise of the Pre-Funded Warrants, to the extent such Pre-Funded Warrants are exercised for cash. We have agreed to pay certain expenses related to the registration of the offer and sale of the shares of Common Stock pursuant to the registration statement of which this prospectus forms a part. The selling stockholders will bear all commissions and discounts, if any, attributable to the sale of the shares.

Our Common Stock is listed on The Nasdaq Global Select Market under the trading symbol “CGEM.” On May 14, 2024, the last reported price of our Common Stock was $26.38 per share.

We may amend or supplement this prospectus from time to time by filing amendments or supplements as required. You should read the entire prospectus and any amendments or supplements carefully before you make your investment decision.

Investing in our securities involves a number of risks. See “Risk Factors” on page 7 and other risk factors contained in the documents incorporated by referenced herein before you make your investment decision.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus or any accompanying prospectus supplement is truthful or complete. Any representation to the contrary is a criminal offense.

The date of this prospectus is June 4, 2024.

TABLE OF CONTENTS

ABOUT THIS PROSPECTUS

This prospectus is part of a registration statement that we filed with the Securities and Exchange Commission, which we refer to as the “SEC,” utilizing a “shelf” registration process.

Under this shelf registration process, the selling stockholders may offer and sell from time to time up to (i) 14,421,070 shares of Common Stock and (ii) 315,790 shares of Common Stock issuable upon the exercise of the Pre-Funded Warrants in one or more offerings. In some cases, the selling stockholders may also be required to provide a prospectus supplement containing specific information about the selling stockholders and the terms on which they are offering and selling the Common Stock. We may also add, update or change in a prospectus supplement or free writing prospectus information contained in this prospectus. You should read this prospectus, any accompanying prospectus supplement, any free writing prospectus and any documents incorporated by reference, as well as any post-effective amendments to the registration statement of which this prospectus is a part, before you make any investment decision. To the extent there is a conflict between the information contained in this prospectus, any applicable prospectus supplement, and any free writing prospectus, including the information incorporated by reference, you should rely on the information in the applicable prospectus supplement. The offer and sale of securities under this prospectus may be made from time to time, in one or more offerings, in any manner described under the section in this prospectus entitled “Plan of Distribution.”

You should rely only on the information contained in or incorporated by reference in this prospectus, any accompanying prospectus supplement or in any related free writing prospectus filed by us with the SEC. Neither we, nor the selling stockholders, has authorized anyone to provide you with any information other than that contained in or incorporated by reference into this prospectus, any accompanying prospectus supplement and any free writing prospectus prepared by or on behalf of us. We and the selling stockholders take no responsibility for, and can provide no assurance as to the reliability of, any information that others may give you. This prospectus and any accompanying prospectus supplement do not constitute an offer to sell or the solicitation of an offer to buy any securities other than the securities described in this prospectus or such accompanying prospectus supplement or an offer to sell or the solicitation of an offer to buy such securities in any circumstances in which such offer or solicitation is unlawful. You should assume that the information appearing in this prospectus, any prospectus supplement, the documents incorporated by reference and any related free writing prospectus is accurate only as of their respective dates. Our business, financial condition, results of operations and prospects may have changed materially since those dates.

We urge you to read carefully this prospectus, together with the information incorporated herein by reference as described under the heading “Incorporation by Reference,” before buying any of the securities being offered.

Unless the context otherwise indicates, references in this prospectus to the “Company,” “we,” “our” and “us” refer, collectively, to Cullinan Therapeutics, Inc., a Delaware corporation, and its consolidated subsidiaries.

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SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS

This prospectus, any accompanying prospectus supplement, any free writing prospectus and any documents incorporated by reference contain forward-looking statements which are made pursuant to the safe harbor provisions of Section 27A of the Securities Act of 1933, as amended (the “Securities Act”), and Section 21E of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), that involve risks, uncertainties, and other factors that may cause actual results, levels of activity, performance, or achievements to be materially different from the information expressed or implied by these forward-looking statements. All statements, other than statements of historical facts, contained in this prospectus, any accompanying prospectus supplement, any free writing prospectus and any documents incorporated by reference, including statements regarding our strategy, future operations, future financial position, future revenue, projected costs, prospects, plans, objectives of management, and expected market growth are forward-looking statements. The words “anticipate,” “believe,” “continue,” “could,” “estimate,” “expect,” “intend,” “may,” “plan,” “potential,” “predict,” “project,” “should,” “target,” “would,” and similar expressions are intended to identify forward-looking statements, although not all forward-looking statements contain these identifying words.

Any forward-looking statements in this prospectus, any accompanying prospectus supplement, any free writing prospectus and any documents incorporated by reference reflect our current views with respect to future events or to our future financial performance and involve known and unknown risks, uncertainties, and other factors that may cause our actual results, performance or achievements to be materially different from any future results, performance or achievements expressed or implied by these forward-looking statements. Factors that may cause actual results to differ materially from current expectations include, among other things, those listed in our Annual Report on Form 10-K for the year ended December 31, 2023 (the “2023 10-K”) and other filings with the Securities Exchange Commission (the “SEC”), including the following:

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These factors are discussed more fully in the 2023 10-K and elsewhere in this prospectus, any accompanying prospectus supplement, any free writing prospectus and any documents incorporated by reference. We may not actually achieve the plans, intentions, or expectations disclosed in our forward-looking statements, and investors should not place undue reliance on our forward-looking statements. Actual results or events could differ materially from the plans, intentions, and expectations disclosed in the forward-looking statements we make. Our forward-looking statements do not reflect the potential impact of any future acquisitions, mergers, dispositions, joint ventures, or investments we may make or collaborations or strategic partnerships we may enter into.

You should read this prospectus, any accompanying prospectus supplement, any free writing prospectus and any documents incorporated by reference completely and with the understanding that our actual future results may be materially different from what we expect. We do not assume any obligation to update any forward-looking statements, whether as a result of new information, future events or otherwise, except as required by law.

This prospectus, any accompanying prospectus supplement, any free writing prospectus and any documents incorporated by reference also contain estimates, projections, and other information concerning our industry, our business, and the markets for our product candidates. Information that is based on estimates, forecasts, projections, market research, or similar methodologies is inherently subject to uncertainties and actual events or circumstances may differ materially from events and circumstances that are assumed in this information. Unless otherwise expressly stated, we obtained this industry, business, market, and other data from our own internal estimates and research, as well as from reports, research surveys, studies, and similar data prepared by market research firms and other third parties, industry, medical and general publications, government data, and similar sources. While we are not aware of any misstatements regarding any third-party information presented in this prospectus, any accompanying prospectus supplement, any free writing prospectus and any documents incorporated by reference, their estimates, in particular, as they relate to projections, involve numerous assumptions, are subject to risks and uncertainties and are subject to change based on various factors, including those discussed under the section titled “Risk Factors” in the 2023 10-K and elsewhere in this prospectus, any accompanying prospectus supplement, any free writing prospectus and any documents incorporated by reference.

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ABOUT THE COMPANY

Overview

We are a clinical-stage biopharmaceutical company dedicated to creating new standards of care for patients. Our strategy is to identify high-impact targets, which we define as those that inhibit key drivers of disease or harness the immune system to eliminate diseased cells in both oncology and autoimmune diseases, and then select what we believe is the optimal therapeutic modality for those targets. We source innovation both internally and externally, focusing on product candidates with novel technology or differentiated mechanisms. Before we advance a product candidate into clinical development, we evaluate its potential for activity as a single agent as well as its ability to generate an immune response or to inhibit disease processes. Using this strategy, we have built a broad and deep pipeline of targeted oncology and autoimmune programs that includes six distinct clinical-stage product candidates.

Academic and industry groups have generated clinical data and case series demonstrating that CD19 targeted CAR-T cell therapy could lead to sustained improvements in disease manifestations, including durable remission from symptoms in some cases, in multiple autoimmune disease indications including systemic lupus erythematosus (“SLE”). Nonetheless, cell therapy has many limitations, including the need for lymphodepleting chemotherapy, which is associated with significant toxicities, an FDA-recognized risk of secondary malignancies, and extended manufacturing lead times. More recently, academic investigators have published data demonstrating the potential for a CD19xCD3 TCE to achieve similar outcomes in multiple autoimmune disease indications. Based on these emerging clinical data supporting the efficacy of CD19 directed therapy in multiple autoimmune diseases and our belief that CLN-978 may address the limitations of CAR-T therapy, we are exploring the development of CLN-978 in a variety of autoimmune diseases and are committed to assessing its broad potential. We plan to submit an IND application to evaluate CLN-978 in patients with SLE as a first indication in the third quarter of 2024.

Clinical observations from three patients treated in a Phase 1 dose escalation trial of patients with relapsed/refractory B-NHL show that CLN-978 was clinically active at the initial starting dose of 30 micrograms administered subcutaneously once weekly. Two of the three patients experienced objective clinical benefit, including one patient who experienced a complete response. Grade 1 cytokine release syndrome occurred in two patients following the first dose of CLN-978 only, and no patients experienced immune effector cell-associated neurotoxicity syndrome. Other adverse events were mostly low-grade and/or mechanistically based (e.g., transient lymphopenia after the first dose only). Of the two patients with detectable B cells at baseline, both experienced rapid, deep, and sustained B

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cell depletion after administration of CLN-978. These data support that CLN-978 can not only deplete peripheral blood B cells but also demonstrate clinical activity in a tissue resident disease at a dose with a favorable safety profile.

In addition to the product candidates described above, we are developing several preclinical oncology programs, all in the discovery stage, including our collaboration with Icahn School of Medicine at Mount Sinai for the development of novel hematopoietic progenitor kinase 1 degraders.

Company Information

Our principal executive offices are located at One Main Street, Suite 1350, Cambridge, Massachusetts 02142, and our telephone number is (617) 410-4650. Our website is located at https://www.cullinantherapeutics.com. The information contained in, or that can be accessed through, our website is not incorporated by reference into this prospectus supplement and should not be considered to be a part of this prospectus supplement. Our website address is included as an inactive textual reference only.

We own or have rights to trademarks, service marks and trade names that we use in connection with the operation of our business, including our corporate name, logos and website names. Other trademarks, service marks and trade names appearing in this prospectus supplement are the property of their respective owners. Solely for convenience, some of the trademarks, service marks and trade names referred to in this prospectus supplement are listed without the ^® and ^™ symbols, but we will assert, to the fullest extent under applicable law, our rights to our trademarks, service marks and trade names.

Implications of Being an Emerging Growth Company and a Smaller Reporting Company

We are an “emerging growth company” as defined in the Jumpstart Our Business Startups Act, or the JOBS Act, enacted in April 2012. As a result, we may take advantage of reduced reporting requirements that are

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otherwise applicable to public companies, including delaying auditor attestation of internal control over financial reporting, providing only two years of audited financial statements and related Management’s Discussion and Analysis of Financial Condition and Results of Operations incorporated by reference in this prospectus supplement and reducing executive compensation disclosures.

We will remain an emerging growth company until the earlier of (1) the last day of the fiscal year (a) following the fifth anniversary of the closing of our initial public offering, (b) in which we have total annual gross revenue of at least $1.235 billion or (c) in which we are deemed to be a large accelerated filer, which requires the market value of our Common Stock that is held by non-affiliates to exceed $700 million as of the prior June 30th, and (2) the date on which we have issued more than $1 billion in non-convertible debt during the prior three-year period.

We are also a “smaller reporting company,” meaning that the market value of our stock held by non-affiliates is less than $700 million and our annual revenue was less than $100 million during our most recently completed fiscal year. We may continue to be a smaller reporting company if either (i) the market value of our stock held by non-affiliates is less than $250 million or (ii) our annual revenue was less than $100 million during the most recently completed fiscal year and the market value of our stock held by non-affiliates is less than $700 million. If we are a smaller reporting company at the time we cease to be an emerging growth company, we may continue to rely on exemptions from certain disclosure requirements that are available to smaller reporting companies. For so long as we remain a smaller reporting company, we are permitted and intend to rely on exemptions from certain disclosure and other requirements that are applicable to other public companies that are not smaller reporting companies.

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RISK FACTORS

Investing in our Common Stock involves a high degree of risk. You should carefully consider the risks and uncertainties described below, and the risk factors set forth under “Risk Factors” in our most recent Annual Report on Form 10-K and other filings we make with the SEC from time to time, which are incorporated by reference in this prospectus, together with all other information included or incorporated by reference in this prospectus, any prospectus supplement, any free writing prospectus, and the information and documents incorporated by reference herein and therein in connection with this offering, before making an investment decision. The risks and uncertainties described below may not be the only ones we face. If any of the risks actually occur, our business, financial condition, operating results, cash flows and prospects could be materially and adversely affected. In that event, the market price of our Common Stock could decline, and you could lose part or all of your investment.

Risks Related to This Offering

The number of shares being registered for sale is significant in relation to the number of outstanding shares of our Common Stock.

We have filed a registration statement of which this prospectus is a part to register the shares offered hereunder for sale into the public market by the selling stockholders. Upon registration of the Common Stock offered hereunder, 14,736,860 shares of the Common Stock registered hereunder may be resold in the public market immediately without restriction. These shares represent a large number of shares of Common Stock, and if sold in the market all at once or at about the same time, could depress the market price of the Common Stock during the period the registration statement remains effective and could also affect our ability to raise equity capital.

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DESCRIPTION OF THE PRIVATE PLACEMENT OF COMMON STOCK AND PRE-FUNDED WARRANTS

On April 18, 2024, we completed a private placement financing transaction (the “Closing”) pursuant to a Stock Purchase Agreement (the “Purchase Agreement”) between us and the selling stockholders named in this prospectus (the “Purchasers”), in which we issued (i) 14,421,070 shares (the “Shares”) of Common Stock and (ii) the Pre-Funded Warrants to purchase 315,790 shares of Common Stock (the “Pre-Funded Warrant Shares”). The Shares and Pre-Funded Warrants (collectively, the “Securities”) were sold at a purchase price of $19.00 per unit and $18.999 per unit, respectively, for aggregate gross cash proceeds of approximately $280 million, before deducting fees to the placement agent and other estimated offering expenses payable by us. The Purchasers of the Securities were the selling stockholders named in this prospectus.

The description of our capital stock is incorporated by reference to Exhibit 4.3 of our Annual Report on Form 10-K for the year ended December 31, 2020 filed with the SEC on March 30, 2021. 

Pre-Funded Warrants

The material terms and provisions of the Pre-Funded Warrants to purchase shares of Common Stock are summarized below. This summary is subject to and qualified in its entirety by the form of pre-funded warrant, which was filed with the SEC as an exhibit to our Current Report on Form 8-K on April 16, 2024.

Exercisability. The holder may exercise the Pre-Funded Warrants at any time or from time to time through April 17, 2054. As further described under “Selling Stockholders,” notwithstanding the foregoing, certain holders will be prohibited from exercising the Pre-Funded Warrants into shares of our Common Stock if, as a result of such exercise, the holder, together with its affiliates, would own more than a specific percentage of the total number of shares of our Common Stock then issued and outstanding. The Pre-Funded Warrants are exercisable, at the option of each holder, in whole or in part, by delivering to us a duly executed exercise notice accompanied by payment in full for the number of shares of our Common Stock purchased upon such exercise (except in the case of a cashless exercise as discussed below).

Exercise Price. The exercise price upon exercise of each Pre-Funded Warrant is $0.001 per share of Common Stock, with an aggregate exercise price of $19.00 per share of Common Stock, of which $18.999 per share of Common Stock was paid by the holder at the Closing. The exercise price of the Pre-Funded Warrants is subject to appropriate adjustment in the event of stock dividends, subdivisions, stock splits, stock combinations, cash distributions, reclassifications, exchanges, combinations or substitutions affecting our Common Stock.

Payment of Exercise Price. The Pre-Funded Warrant holders have the option to provide payment of the exercise price of the shares being acquired upon exercise of the warrants (i) by check, wire transfer of same day funds, or other form of payment acceptable to us, or (ii) by cashless exercise.

Cashless Exercise. In lieu of making the cash payment otherwise contemplated to be made to us upon such exercise in payment of the aggregate exercise price, the Pre-Funded Warrant holders may elect instead to receive upon such exercise the net number of shares of Common Stock determined according to a formula set forth in the Pre-Funded Warrant.

Transferability. Subject to applicable laws and the restriction on transfer set forth in the Pre-Funded Warrant, the Pre-Funded Warrants may be transferred.

Acquisitions. If, at any time while each Pre-Funded Warrant is outstanding, there is an acquisition, which includes (i) the disposition of all or substantially all of our assets, (ii) a merger or consolidation in which our stockholders immediately prior to such merger, consolidation or reorganization, own less than a majority of our outstanding voting power immediately after such merger, consolidation or reorganization, the sale of all or

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substantially all of our assets or voting securities, or other change of control transaction, or (iii) any sale or other transfer by our stockholders of shares representing at least a majority of our then-total outstanding combined voting power, then the holders shall receive upon exercise of the Pre-Funded Warrants the kind and amount of securities, cash or other property which the holder would have been entitled to receive pursuant to such acquisition if such exercise had taken place immediately prior to such acquisition.

Rights as a Stockholder. Except for the right to participate in certain dividends and distributions and as otherwise provided in the Pre-Funded Warrants or by virtue of a holder’s ownership of our Common Stock, the holders of the Pre-Funded Warrants do not have the rights or privileges of holders of our Common Stock, including any voting rights, until they exercise their Pre-Funded Warrants.

Waivers and Amendments. The Pre-Funded Warrants and any term thereof may only be amended or otherwise changed, waived, discharged or terminated in writing, signed by the Company and at least a majority in interest of the aggregate number of shares of Common Stock then issuable (without regards to any exercise limitations) upon exercise of the then outstanding Pre-Funded Warrants issued under the Purchase Agreement, provided that no amendment to the exercise price of the Pre-Funded Warrant, expiration date of the Pre-Funded Warrant, the provisions on limitations on exercise of the Pre-Funded Warrant, the provisions on the term or automatic cashless exercise upon expiration of the Pre-Funded Warrant or the amendment provision to the Pre-Funded Warrant may be made without consent of the holder.

No Fractional Shares. No fractional shares shall be issued upon the exercise of the Pre-Funded Warrants, and the number of Pre-Funded Warrant Shares to be issued shall be rounded down to the nearest whole share. If a fractional Pre-Funded Warrant Share interest arises upon any exercise of the Pre-Funded Warrant, we shall eliminate such fractional Pre-Funded Warrant Share interest by paying the holder in cash the amount computed by multiplying the fractional interest by (i) the fair market value (as determined in accordance with the Pre-Funded Warrant) of a full Pre-Funded Warrant Share, less (ii) the then-effective exercise price.

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USE OF PROCEEDS

The selling stockholders will receive all net proceeds from the sale of Common Stock pursuant to this prospectus. However, in the case of the Pre-Funded Warrants issued to the selling stockholders pursuant to the Purchase Agreement, upon any exercise of the Pre-Funded Warrants for cash, the selling stockholders would pay us an exercise price of $0.001 per share of Common Stock, subject to any adjustment pursuant to the terms of the Pre-Funded Warrants. We intend to use any such proceeds for general corporate purposes.

The selling stockholders will pay any underwriting discounts and commissions and expenses incurred by them for brokerage, accounting, tax or legal services or any other expenses incurred in disposing of the securities. We will bear the costs, fees and expenses incurred in effecting the registration of the shares covered by this prospectus, including all registration and filing fees, Nasdaq listing fees and fees and expenses of our counsel and our independent registered public accounting firm.

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SELLING STOCKHOLDERS

Pursuant to the Registration Rights Agreement, dated April 15, 2024, by and between us and the selling stockholders (the “Registration Rights Agreement”), we agreed to file the registration statement of which this prospectus is a part to cover the resale of the shares of our Common Stock and the shares of Common Stock underlying the Pre-Funded Warrants, which the selling stockholders acquired pursuant to the Purchase Agreement, and to keep such registration statement effective with respect to each selling stockholder until (i) the fifth anniversary of the date that such registration statement is declared effective, (ii) such date that all Registrable Securities (as such term is defined in the Registration Rights Agreement) covered by such registration statement cease to be Registrable Securities, (iii) such date that all Registrable Securities covered by such registration statement become eligible for resale without volume or manner-of-sale restrictions pursuant to Rule 144 as promulgated by the SEC under the Securities Act (“Rule 144”) and without the requirement for us to be in compliance with the current public information requirement under Rule 144 or (iv) such date that all Registrable Securities have been sold pursuant to a registration statement under the Securities Act or under Rule 144. The shares of Common Stock issuable upon exercise of the Pre-Funded Warrants will become eligible for sale by the selling stockholders under this prospectus only when the Pre-Funded Warrants are exercised. We cannot predict when or whether any of the selling stockholders will exercise their Pre-Funded Warrants.

We are registering the resale of the above-referenced shares to permit each of the selling stockholders identified below, or their transferees, pledgees, donees or successors that may be identified in a supplement to this prospectus or, if required, a post-effective amendment to the registration statement of which this prospectus is a part, to resell or otherwise dispose of the shares in the manner contemplated under “Plan of Distribution” in this prospectus (as may be supplemented and amended). This prospectus covers the sale or other disposition by the selling stockholders of up to the total number of shares of Common Stock that may be issued to the selling stockholders pursuant to the Purchase Agreement, plus the total number of shares of Common Stock issuable upon exercise of the Pre-Funded Warrants issued to the selling stockholders. Throughout this prospectus, when we refer to the shares of Common Stock being registered on behalf of the selling stockholders, we are referring to the shares of Common Stock and the shares underlying the Pre-Funded Warrants issued to the selling stockholders pursuant to the Purchase Agreement, and when we refer to the selling stockholders in this prospectus, we are referring to the purchasers under the Purchase Agreement and, as applicable, their transferees, pledgees, donees or successors that may be identified in a supplement to this prospectus or, if required, a post-effective amendment to the registration statement of which this prospectus is a part.

The selling stockholders may sell some, all or none of their shares. We do not know how long the selling stockholders will hold the shares before selling them, and we currently have no agreements, arrangements or understandings with the selling stockholders regarding the sale or other disposition of any of the shares. The shares of Common Stock covered hereby may be offered from time to time by the selling stockholders.

The information regarding shares beneficially owned after the offering assumes the sale of all shares offered by the selling shareholders. To our knowledge, subject to community property laws where applicable, each person named in the table has sole voting and investment power with respect to the shares of Common Stock set forth opposite such person’s name.

The following table sets forth the name of each selling stockholder, the number and percentage of Common Stock beneficially owned by the selling stockholders as of April 29, 2024, the number of shares of Common Stock that may be offered under this prospectus, and the number and percentage of Common Stock beneficially owned by the selling stockholders assuming all of the shares of Common Stock registered hereunder are sold. Beneficial ownership is determined in accordance with the rules of the SEC and includes voting or investment power with respect to the Common Stock. Generally, a person “beneficially owns” shares of the Common Stock if the person has or shares with others the right to vote those shares or to dispose of them, or if the person has the right to acquire voting or disposition rights within 60 days of April 29, 2024. The number of shares of Common Stock in the column “Number of Shares Offered” represents all of the shares of Common Stock that a selling

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stockholder may offer and sell from time to time under this prospectus. The number of shares of Common Stock into which the Pre-Funded Warrants beneficially held by each selling stockholder are exercisable are limited pursuant to the terms of the Pre-Funded Warrants to a number of shares of Common Stock which would result in such selling stockholder having aggregate individual beneficial ownership of the total issued and outstanding shares of Common Stock over specified percentage thresholds (collectively, referred to as the “Beneficial Ownership Limitation”).

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Based on 57,556,714 shares of Common Stock outstanding as of April 29, 2024.

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PLAN OF DISTRIBUTION

The selling securityholders, which as used herein shall include donees, pledgees, assignees, transferees or other successors-in-interest selling the securities or interests in such securities received after the date of this prospectus from a selling securityholder as a gift, pledge, partnership distribution or other transfer, may, from time to time, sell, transfer or otherwise dispose of any or all of their shares of Common Stock or interests therein on any stock exchange, market or trading facility on which the securities are traded or in private transactions. These dispositions may be at fixed prices, through distributions in kind for no consideration, at prevailing market prices at the time of sale, at prices related to the prevailing market price, at varying prices determined at the time of sale, or at negotiated prices.

The selling securityholders may use any one or more of the following methods when disposing of the securities or interests therein:

The selling securityholders may, from time to time, pledge or grant a security interest in some or all of the securities owned by them and, if they default in the performance of their secured obligations, the pledgees or secured parties may offer and sell the securities, from time to time, under this prospectus, or under an amendment to this prospectus under Rule 424(b) or other applicable provision of the Securities Act amending the list of selling securityholders to include the pledgee, transferee or other successors in interest as selling securityholders under this prospectus. The selling securityholders also may transfer the securities in other circumstances, in which case the pledgees, transferees or other successors in interest will be the selling beneficial owners for purposes of this prospectus.

In connection with the sale of our securities or interests therein, the selling securityholders may enter into hedging transactions with broker-dealers or other financial institutions, which may in turn engage in short sales of the securities in the course of hedging the positions they assume. The selling securityholders may also sell securities short and deliver these securities to close out their short positions, or loan or pledge the securities to broker-dealers that in turn may sell these securities. The selling securityholders may also enter into options or other transactions with broker-dealers or other financial institutions or the creation of one or more derivative securities which require the delivery to each such broker-dealer or other financial institution of the securities

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offered by this prospectus, which securities such broker-dealer or other financial institution may resell pursuant to this prospectus (as supplemented or amended to reflect such transaction).

The aggregate proceeds to the selling securityholders from the sale of the securities offered by them will be the purchase price of the securities less discounts or commissions, if any. Each of the selling securityholders reserves the right to accept and, together with its agents from time to time, to reject, in whole or in part, any proposed purchase of the securities to be made directly or through agents. We will not receive any of the proceeds from this offering.

Upon any exercise of the pre-funded warrants by payment of cash, however, we will receive the exercise price of the pre-funded warrants.

The selling securityholders also may resell all or a portion of the securities in open market transactions in reliance upon Rule 144 under the Securities Act, provided that they meet the criteria and conform to the requirements of that rule.

The selling securityholders and any underwriters, broker-dealers or agents that participate in the sale of the securities or interests therein may be “underwriters” within the meaning of Section 2(a)(11) of the Securities Act. Any discounts, commissions, concessions or profit they earn on any resale of the securities may be underwriting discounts and commissions under the Securities Act. Selling securityholders who are “underwriters” within the meaning of Section 2(a)(11) of the Securities Act will be subject to the prospectus delivery requirements of the Securities Act.

To the extent required, the securities to be sold, the names of the selling securityholders, the respective purchase prices and public offering prices, the names of any agents, dealer or underwriter and any applicable commissions or discounts with respect to a particular offer will be set forth in an accompanying prospectus supplement or, if appropriate, a post-effective amendment to the registration statement that includes this prospectus.

In order to comply with the securities laws of some states, if applicable, the securities may be sold in these jurisdictions only through registered or licensed brokers or dealers. In addition, in some states the securities may not be sold unless it has been registered or qualified for sale or an exemption from registration or qualification requirements is available and is complied with.

We have advised the selling securityholders that the anti-manipulation rules of Regulation M under the Exchange Act may apply to sales of the securities in the market and to the activities of the selling securityholders and their affiliates. In addition, to the extent applicable we will make copies of this prospectus (as it may be supplemented or amended from time to time) available to the selling securityholders for the purpose of satisfying the prospectus delivery requirements of the Securities Act. The selling securityholders may indemnify any broker-dealer that participates in transactions involving the sale of the securities against certain liabilities, including liabilities arising under the Securities Act.

We have agreed to indemnify the selling securityholders against liabilities, including liabilities under the Securities Act and state securities laws, relating to the registration of the shares of Common Stock offered by this prospectus.

We have agreed with the selling securityholders to use commercially reasonable efforts to keep the registration statement of which this prospectus constitutes a part continuously effective until the earlier of (1) such time as all of the securities covered by this prospectus have been disposed of pursuant to and in accordance with the registration statement or (2) the date on which all of the securities may be sold without restriction pursuant to Rule 144 of the Securities Act. We have advised the selling securityholders that the anti-manipulation rules of Regulation M under the Exchange Act may apply to sales of securities in the market and to

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the activities of the selling securityholders and their affiliates. In addition, to the extent applicable, we will make copies of this prospectus (as it may be supplemented or amended from time to time) available to the selling securityholders for the purpose of satisfying the prospectus delivery requirements of the Securities Act. The selling securityholders may indemnify any broker-dealer that participates in transactions involving the sale of the securities against certain liabilities, including liabilities arising under the Securities Act.

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LEGAL MATTERS

The validity of any securities offered from time to time by this prospectus will be passed upon by Ropes & Gray LLP.

EXPERTS

The consolidated financial statements of Cullinan Therapeutics, Inc. (formerly known as Cullinan Oncology, Inc.) as of December 31, 2023 and 2022, and for the years then ended, have been incorporated by reference herein and in the registration statement in reliance upon the report of KPMG LLP, independent registered public accounting firm, incorporated by reference herein, and upon the authority of said firm as experts in accounting and auditing.

WHERE YOU CAN FIND MORE INFORMATION

We file annual, quarterly and current reports, proxy statements and other information with the SEC. Our SEC filings are available to the public over the Internet at the SEC’s website at https://www.sec.gov. Copies of certain information filed by us with the SEC are also available on our website at https://www.cullinantherapeutics.com. Our website is not a part of this prospectus and is not incorporated by reference in this prospectus.

This prospectus is part of a registration statement we filed with the SEC. This prospectus omits some information contained in the registration statement in accordance with SEC rules and regulations. You should review the information and exhibits in the registration statement for further information about us and our consolidated subsidiaries and the securities we are offering. Statements in this prospectus concerning any document we filed as an exhibit to the registration statement or that we otherwise filed with the SEC are not intended to be comprehensive and are qualified by reference to these filings and the exhibits attached thereto. You should review the complete document to evaluate these statements.

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INCORPORATION BY REFERENCE

The SEC allows us to incorporate by reference much of the information we file with the SEC, which means that we can disclose important information to you by referring you to those publicly available documents. The information that we incorporate by reference in this prospectus is considered to be part of this prospectus. Because we are incorporating by reference future filings with the SEC, this prospectus is continually updated and those future filings may modify or supersede some of the information included or incorporated in this prospectus. This means that you must look at all of the SEC filings that we incorporate by reference to determine if any of the statements in this prospectus or in any document previously incorporated by reference have been modified or superseded. This prospectus incorporates by reference the documents listed below (File No. 001-39856) and any future filings we make with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act (in each case, other than those documents or the portions of those documents not deemed to be filed) between the date of the initial registration statement and the effectiveness of the registration statement and following the effectiveness of the registration statement until the offering of the securities under the registration statement is terminated or completed:

You may request a copy of these filings, at no cost, by writing or telephoning us at the following address or telephone number:

Cullinan Therapeutics, Inc.

One Main Street

Suite 1350

Cambridge, Massachusetts 02142

Attn: Investor Relations

(617) 410-4650

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