UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

PROXY STATEMENT PURSUANT TO SECTION 14(a) OF THE

SECURITIES EXCHANGE ACT OF 1934

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IGM Biosciences, Inc.

(Name of Registrant as Specified In Its Charter)

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Your Vote Counts! IGM BIOSCIENCES, INC. 2022 Annual Meeting Vote by June 20, 2022 11:59 PM ET IGM BIOSCIENCES, INC. 325 E. MIDDLEFIELD ROAD MOUNTAIN VIEW, CA 94043 D85477-P75256 You invested in IGM BIOSCIENCES, INC. and it’s time to vote! You have the right to vote on proposals being presented at the Annual Meeting. This is an important notice regarding the availability of proxy material for the stockholder meeting to be held on June 21, 2022. Get informed before you vote View the Notice & Proxy Statement and Form 10-K online OR you can receive a free paper or email copy of the material(s) by requesting prior to June 7, 2022. If you would like to request a copy of the material(s) for this and/or future stockholder meetings, you may (1) visit www.ProxyVote.com, (2) call 1-800-579-1639 or (3) send an email to sendmaterial@proxyvote.com. If sending an email, please include your control number (indicated below) in the subject line. Unless requested, you will not otherwise receive a paper or email copy. For complete information and to vote, visit www.ProxyVote.com Control # Smartphone users Vote Virtually at the Meeting* Point your camera here and June 21, 2022 8:00 a.m. (Pacific Time) vote without entering a control number Virtually at: www.virtualshareholdermeeting.com/IGMS2022 *Please check the meeting materials for any special requirements for meeting attendance.

Vote at www.ProxyVote.com THIS IS NOT A VOTABLE BALLOT This is an overview of the proposals being presented at the upcoming stockholder meeting. Please follow the instructions on the reverse side to vote these important matters. Voting Items Board Recommends 1. Election of Directors Nominees: 01) Felix J. Baker, Ph.D. For 02) Michael Lee 03) Fred Schwarzer 2. Ratification of the appointment of Deloitte & Touche LLP as our independent registered public accounting firm for our For fiscal year ending December 31, 2022. NOTE: The proxy holders will vote in their discretion on any other business as may properly come before the Annual Meeting or any adjournment orpostponement thereof. Prefer to receive an email instead? While voting on www.ProxyVote.com, be sure to click “Sign up for E-delivery”.