Equillium Inc美股招股说明书(2025-09-19版)

Up to $75,000,000 Common Stock This prospectus supplement (the “Prospectus Supplement”) amends and supplements the information in the prospectus dated January18, 2023(the “Prior Prospectus”), filed with the Securities and Exchange Commission (the “SEC”) as part of our registration statement on FormS-3(FileNo.333-269153)(the “Registration Statement”), as previously amended and supplemented by our prospectus supplements dated October5, 2023 andFebruary23, 2024 (together with the Prior Prospectus, the “ATM Prospectus”), relating to the offer and sale of shares of our common stock, $0.0001 parvalue per share, having an aggregate offering price of up to $21,950,000 pursuant to the terms of that certain Open Market Sale Agreement (the“Original Sales Agreement”) dated October5, 2023, with Jefferies LLC (“Jefferies”). On August3, 2025, we entered into that certain Amendment No.1to the Original Sales Agreement (as amended, the “Sales Agreement”), pursuant to which Jefferies was replaced as the sales agent by LifeSci CapitalLLC (the “Sales Agent”). As disclosed in our Current Report on Form8-Kfiled with the SEC on August4, 2025, all references to Jefferies in the ATMProspectus shall now refer to the Sales Agent. This Prospectus Supplement should be read in conjunction with the ATM Prospectus, and is qualified byreference thereto, except to the extent that the information herein amends or supersedes the information contained in the ATM Prospectus. ThisProspectus Supplement is not complete without, and may only be delivered or utilized in connection with, the ATM Prospectus, and any futureamendments or supplements thereto. As of the date hereof, the aggregate market value of our outstanding common stock held bynon-affiliates(our “public float”), calculated inaccordance with General Instruction I.B.6 of FormS-3under the Securities Act of 1933, as amended, was approximately $93.7million, which wascalculated based on 47,085,662 shares of our outstanding common stock held bynon-affiliatesas of September4, 2025, at a price of $1.99 per share asof September11, 2025, which was the highest closing price of our common stock on The Nasdaq Capital Market within 60 days of the filing of thisProspectus Supplement. We are filing this Prospectus Supplement to amend the ATM Prospectus to update our public float, and indicate that, since our public float isabove $75.0million, we are no longer subject to the sales limitations of General Instruction I.B.6 of FormS-3with respect to the Registration Statementof which the ATM Prospectus, as amended by this Prospectus Supplement, forms a part. If we become subject to the limitations of General InstructionI.B.6 in the future, we will file another supplement to the ATM Prospectus, as amended by this Prospectus Supplement. Since our entry into the Sales Agreement, we have sold approximately $0.96million of common stock pursuant to the Sales Agreement.Accordingly, because we are no longer subject to the sales limitations of General Instruction I.B.6, we may offer and sell up to an additional$74.0million of shares of our common stock pursuant to the Sales Agreement from time to time through the Sales Agent. Our common stock trades on the Nasdaq Capital Market under the symbol “EQ.” On September18, 2025, the last reported sale price of ourcommon stock on the Nasdaq Capital Market was $1.50per share. Investing in our securities involves a high degree of risk. Before making an investment decision, please read theinformation under “Risk Factors” on pageS-7of the Prior Prospectus and in the documents incorporated by referenceinto this Prospectus Supplement and the ATM Prospectus, and under similar headings in the other documents that arefiled after the date hereof and incorporated by reference into this Prospectus Supplement and the ATM Prospectus. LifeSci Capital The date of this Prospectus Supplement is September19, 2025.