ClearSign Technologies Corp 美股招股说明书（2026年6月1日版）

777,780Shares of Common Stock We are offering shares of our common stock, par value $0.0001 per share. Our common stock is listed on the Nasdaq Capital Market under the symbol “CLIR.” On May 27, 2026, the last reported saleprice of our common stock on the Nasdaq Capital Market was $4.45 per share. We have granted the underwriter an option for a period of up to 30 days from the date of this prospectus supplement topurchase up to an additional 116,667 shares of our common stock at the public offering price, less the underwriting discounts and As of the date hereof, the aggregate market value of shares of our common stock held by non-affiliates, or the public float, is$36,786,497, which was calculated based on 5,300,648 shares of our common stock outstanding held by non-affiliates at a price of$6.94 per share, the closing price of our common stock on April 6, 2026. Pursuant to General Instruction I.B.6 of FormS-3, in noevent will we sell shares pursuant to this prospectus with a value of more than one-third of the public float in any 12-month period, so (1) We have agreed to reimburse the underwriter for certain expenses. See “Underwriting” beginning on page S-13 of this prospectussupplement for additional information regarding underwriter compensation. An investment in our common stock involves a high degree of risk. See “Risk Factors” beginning on page S-8 of thisprospectus supplement, and the risk factors incorporated by reference into this prospectus supplement and the accompanying Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved ofthese securities or passed upon the adequacy or accuracy of this prospectus supplement. Any representation to the contrary isa criminal offense. We anticipate delivery of the shares will be made on or about June 1, 2026, subject to customary closing conditions. Newbridge Securities Corporation The date of this prospectus supplement is May 28, 2026. Prospectus Supplement ABOUT THIS PROSPECTUS SUPPLEMENT This prospectus supplement and the accompanying prospectus form a part of a registration statement on FormS-3 (FileNo.333-288736), which was declared effective on July 28, 2025, that we filed with the Securities and Exchange Commission, or the This document is in two parts. The first part is the prospectus supplement, including the documents incorporated by reference,which describes the specific terms of this offering. The second part, the accompanying prospectus, including the documentsincorporated by reference, provides more general information. Generally, when we refer to this prospectus, we are referring to bothparts of this document combined. We urge you to carefully read this prospectus supplement and the accompanying prospectus, and thedocuments incorporated by reference herein and therein, before buying any of the securities being offered under this prospectussupplement. This prospectus supplement may add, update or change information contained in the accompanying prospectus. To the You should rely only on the information contained in this prospectus supplement, the accompanying prospectus and anyrelated free-writing prospectus that we or the underwriters provide to you or incorporated herein by reference in this prospectussupplement and contained or incorporated therein by reference in the accompanying prospectus. Neither we nor the underwriters have We are offering to sell the securities only in jurisdictions where such offers and sales are permitted. The distribution of thisprospectus supplement and the accompanying prospectus and the offering of the securities in certain jurisdictions or to certain personswithin such jurisdictions may be restricted by law. Persons outside the United States who come into possession of this prospectussupplement and the accompanying prospectus must inform themselves about and observe any restrictions relating to the offering of thesecurities and the distribution of this prospectus supplement and the accompanying prospectus outside the United States. This You should assume that the information in this prospectus supplement and the accompanying prospectus is accurate only asof the date on the front of the applicable document and that any information we have incorporated by reference is accurate only as ofthe date of the document incorporated by reference, regardless of the time of delivery of this prospectus supplement or theaccompanying prospectus, or any sale of a security. Our business, financial condition, results of operations and prospects may havechanged since those dates. You should read this prospectus supplement, the accompanying prospectus and the documents incorporated The representations, warranties and covenants made by us in any agreement that is filed as an exhibit to any document that isincorporated by reference herein or in the accompanying prospectus were made solely for the benefit of the parties to such agreement,including, in some cases, fo