ClearSign Technologies Corporation

Item 1.01. Entry into a Material Definitive Agreement. On May 28, 2026, ClearSign Technologies Corporation (the “Company”) entered into an underwriting agreement (the “Underwriting Agreement”) with Newbridge Securities Corporation (the “Underwriter”), relating to a firm-commitment underwritten public offering (the “Offering”), for the issuance and sale to primarily existing stockholders of the Company of 777,780 shares (the “Firm Shares”) of the Company’s common stock, par value $0.0001 per share (the “Common Stock”), at a public offering price of $4.33 per Firm Share, less underwriting discounts and commissions, pursuant to an effective registration statement on Form S-3 (File No. 333-288736) (the “Registration Statement”), including the prospectus forming a part of the Registration Statement, as supplemented by a preliminary prospectus supplement, dated May 28, 2026, and a final prospectus supplement, dated May 28, 2026, each filed with the Securities and Exchange Commission. Under the terms of the Underwriting Agreement, the Company also granted the Underwriter an option exercisable for thirty (30) days to purchase up to an additional 116,667 shares of Common Stock (the “Additional Shares,” and together with the Firm Shares, the “Public Securities”) from the Company at the Firm Share price, less underwriting discounts and commissions, to cover over-allotments. The Company expects the net proceeds from the Offering to be approximately $2.94 million after deducting underwriting discounts and commissions and estimated Offering expenses. The Company intends to use the net proceeds from the Offering for working capital, research and development, marketing and sales, and general corporate purposes. The Underwriting Agreement contains customary representations, warranties and agreements by the Company, customary conditions to closing, indemnification obligations of the Company and the Underwriter, including for liabilities under the Securities Act of 1933, as amended, various other obligations of the parties, and termination provisions. In addition, under the Underwriting Agreement, the Company agreed, subject to certain exceptions, not to offer, pledge, sell, contract to sell, sell any option, right or warrant to purchase, lend or otherwise transfer or dispose, directly or indirectly, any shares of Common Stock or any securities convertible into or exercisable or exchangeable for shares of Common Stock, for a period of ninety (90) days from the date of the final prospectus supplement relating to the Offering, without the prior written consent of the Underwriter; provided, however, that the Company may sell shares of Common Stock pursuant to that certain At The Market Offering Agreement with H.C. Wainwright & Co., LLC following the date that is thirty (30) days after the date of the Underwriting Agreement. The Offering is expected to close on or about June 1, 2026, subject to the closing conditions contained in the Underwriting Agreement. Pursuant to the Underwriting Agreement, the Company’s executive officers and directors entered into lock-up agreements substantially in the form included as an exhibit to the Underwriting Agreement, under which they agreed, subject to certain exceptions, not to sell, transfer or dispose of, directly or indirectly, any shares of Common Stock or securities convertible into or exercisable or exchangeable for shares of Common Stock for a period of ninety (90) days from the date of the final prospectus supplement relating to the Offering. The above description of the Underwriting Agreement is qualified in its entirety by reference to the full text of the Underwriting Agreement, a copy of which is filed as Exhibit 1.1 hereto and is incorporated herein by reference. Mitchell Silberberg & Knupp LLP, counsel to the Company, has issued an opinion to the Company, dated June 1, 2026, regarding the validity of the Public Securities. A copy of the opinion is filed as Exhibit 5.1 hereto.

Item 8.01 Other Events. On May 28, 2026, the Company issued a press release announcing that it had launched the Offering, and on May 29, 2026, the Company issued a press release announcing that it had priced the Offering. The press releases are filed as Exhibits 99.1 and 99.2 to this Current Report on Form 8-K and are incorporated herein by reference.