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( Mark One ) X ANNUAL REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington , D.C. 20549 TRANSITION REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 For the transition period from WASHINGTON ( State or other jurisdiction of incorporation or organization ) Title of each class FORM 10 - K For the fiscal year ended December 31 , 2020 Common Stock CLEARSIGN TECHNOLOGIES CORPORATION ( Exact name of registrant as specified in its charter ) OR Securities registered pursuant to Section 12 ( b ) of the Act : Large accelerated filer Non - accelerated filer x Commission file number 001-35521 to 12870 Interurban Avenue South Seattle , Washington 98168 ( Address of principal executive offices ) ( Zip Code ) ( 206 ) 673-4848 ( Registrant's telephone number , including area code ) Trading Symbol ( s ) Name of each exchange on which registered The Nasdaq Stock Market LLC Indicate by check mark if the registrant is a well - known seasoned issuer , as defined in Rule 405 of the Securities Act . Yes ¨ No x Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15 ( d ) of the Act . Yes No x Indicate by check mark whether the registrant ( 1 ) has filed all reports required to be filed by Section 13 or 15 ( d ) of the Securities Exchange Act of 1934 during the preceding 12 months ( or for such shorter period that the registrant was required to file such reports ) ; and ( 2 ) has been subject to such filing requirements for the past 90 days . Yes x No¨ 26-2056298 ( I.R.S. Employer Identification No. ) CLIR Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S - T ( § 232.405 of this chapter ) during the preceding 12 months ( or for such shorter period that the registrant was required to submit such files ) . Yes x No " . Indicate by check mark whether the registrant is a large accelerated filer , an accelerated filer , a non - accelerated filer , a smaller reporting company or an emerging growth company . See the definitions of " large accelerated filer , " " accelerated filer , " " smaller reporting company , " and " emerging growth company " in Rule 12b - 2 of the Exchange Act . Accelerated filer Smaller reporting company x Emerging growth company If an emerging growth company , indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13 ( a ) of the Exchange Act . ¨