ClearSign Technologies Corp美股招股说明书(2026-05-28版)

SUBJECT TO COMPLETION, DATED MAY 28, 2026 PRELIMINARY PROSPECTUS SUPPLEMENT(To the Prospectus Dated July 28, 2025) Shares 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 additionalshares of our common stock at the public offering price, less the underwriting discounts andcommissions. 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, solong as the public float is less than $75,000,000. During the 12-calendar month period that ends on, and includes, the date of thisprospectus supplement, we have not sold any securities pursuant to General Instruction I.B.6. of FormS-3. Per shareTotalPublic offering price$$Underwriting discounts and commissions(1)$$Proceeds, before expenses, to us$$ (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 accompanyingprospectus, for more information on these risks. 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, 2026, subject to customary closing conditions. Newbridge Securities Corporation The date of this prospectus supplement is, 2026. TABLE OF CONTENTS Prospectus Supplement PageABOUT THIS PROSPECTUS SUPPLEMENTS-1MARKET, INDUSTRY AND OTHER DATAS-2DISCLOSURE REGARDING FORWARD-LOOKING STATEMENTSS-2PROSPECTUS SUPPLEMENT SUMMARYS-4RISK FACTORSS-8USE OF PROCEEDSS-10DIVIDEND POLICYS-10DILUTIONS-11UNDERWRITINGS-13LEGAL MATTERSS-16EXPERTSS-16WHERE YOU CAN FIND MORE INFORMATIONS-16INFORMATION INCORPORATED BY REFERENCES-17 Prospectus PageABOUT THIS PROSPECTUS1MARKET,INDUSTRY AND OTHER DATA1DISCLOSURE REGARDING FORWARD-LOOKING STATEMENTS1OUR BUSINESS3RISK FACTORS6USE OF PROCEEDS6DILUTION6DESCRIPTION OF SECURITIES THAT MAYBE OFFERED7Description of Debt Securities7Description of Common Stock14Description of Preferred Stock16Description of Warrants18Description of Subscription Rights19Description of Units20PLAN OF DISTRIBUTION20LEGAL MATTERS23EXPERTS23WHERE YOU CAN FIND MORE INFORMATION23INFORMATION INCORPORATED BY REFERENCE25 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 theSEC, utilizing a “shelf” registration process. 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 theextent that any statement that we make in this prospectus supplement is inconsistent with statements made in the accompanyingprospectus or any documents incorporated by reference therein filed prior to the date of this prospectus supplement, the statementsmade in this prospectus supplement will be deemed to modify or supersede those made in the accompanying prospectus and suchdocuments incorporated by reference therein. You should rely only on the information contained in this prospectus supplement, the accompanying prospectu