Supplemental Prospectus (Form 424B3)

Filed Pursuant to Rule 424(b)(3)
Registration No. 333-290108

PROSPECTUS SUPPLEMENT

(To Prospectus dated September 15, 2025)

ImageneBio, Inc.

2,508,337 Shares of Common Stock

This prospectus supplement supplements the prospectus dated September 15, 2025 (the "Prospectus"), which forms a part of our registration statement on Form S-1 (No. 333-290108). This prospectus supplement is being filed to update and supplement the information in the Prospectus with certain information contained in our Current Reports on Form 8-K, filed with the Securities and Exchange Commission on November 5, 2025 (the "Current Report"). Accordingly, we have attached the Current Report in relevant part to this prospectus supplement.

The Prospectus and this prospectus supplement relate to the proposed offer and resale or other disposition from time to time by the selling stockholders identified in this prospectus of 2,508,337 shares of common stock, par value $0.001 per share, (the "Common Stock") of ImageneBio, Inc.

Our Common Stock is listed on the Nasdaq Capital Market under the ticker symbol "IMA." On November 4, 2025, the last reported sales price of our Common Stock was $8.24 per share.

This prospectus supplement should be read in conjunction with the Prospectus, including any amendments or supplements to it, which is to be delivered with this prospectus supplement. This prospectus supplement is qualified by reference to the Prospectus, including any amendments or supplements thereto, except to the extent that the information provided by this prospectus supplement supersedes information contained in the Prospectus.

This prospectus supplement is not complete without, and may not be delivered or used except in conjunction with, the Prospectus, including any amendments or supplements to it.

Investing in our securities involves a high degree of risk. You should review carefully the risks and uncertainties described in the section titled "Risk Factors" beginning on page 6 of the Prospectus, and under similar headings in any amendments or supplements to the Prospectus.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities, or passed upon the accuracy or adequacy of this prospectus supplement. Any representation to the contrary is a criminal offense.

The date of this prospectus supplement is November 5, 2025

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 OR 15(d)

of The Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): November 1, 2025

IMAGENEBIO, INC.

(Exact name of registrant as specified in its charter)

Registrant's telephone number, including area code: (858) 345-6265

Not Applicable

(Former Name or Former Address, if Changed Since Last Report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

Securities registered pursuant to Section 12(b) of the Act:

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).

Emerging growth company ☒

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. ☐

(d)

On November 1, 2025, the Board of Directors (the "Board") of ImageneBio, Inc. (the "Company") increased the authorized size of the Board from six directors to seven directors and, upon recommendation of the Nominating and Corporate Governance Committee of the Board, appointed Joseph P. Slattery to the Board, effective November 1, 2025, and as the Chair of the Audit Committee of the Board, effective November 18, 2025.

The Company has entered into its standard indemnification agreement with Mr. Slattery in the form previously filed as Exhibit 10.10 to the Company's Registration Statement on Form S-1 (No. 333-290108), filed with the SEC on September 8, 2025.

SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.