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( Mark One ) UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington , D.C. 20549 FORM 10 - K ☑ ☐ ANNUAL REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 For the fiscal year ended December 31 , 2020 OR TRANSITION REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 FOR THE TRANSITION PERIOD FROM Delaware ΤΟ Commission File Number 001-38130 Aileron Therapeutics , Inc. ( Exact name of Registrant as specified in its Charter ) ( State or other jurisdiction of incorporation or organization ) 290 Pleasant Street , Unit 112 Watertown , MA ( Address of principal executive offices ) Title of each class Common Stock , $ 0.001 par value 13-4196017 ( I.R.S. Employer Identification No. ) Registrant's telephone number , including area code : ( 617 ) 995-0900 Securities registered pursuant to Section 12 ( b ) of the Act : Trading Symbol ( s ) ALRN 02472 ( Zip Code ) Name of each exchange on which registered The Nasdaq Capital Market Securities registered pursuant to Section 12 ( g ) of the Act : None Indicate by check mark if the Registrant is a well - known seasoned issuer , as defined in Rule 405 of the Securities Act . YES □ NO Indicate by check mark if the Registrant is not required to file reports pursuant to Section 13 or 15 ( d ) of the Act . YES □ NO ☑ 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 NO □ 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 NO □ Indicate by check mark whether the registrant is a large accelerated filer , an accelerated filer , a non - accelerated filer , 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 . Large accelerated filer Non - accelerated filer ☐ ☑ Accelerated filer ☐ Smaller reporting company 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 . ☑ Indicate by check mark whether the registrant has filed a report on and attestation to its management's assessment of the effectiveness of its internal control over financial reporting under Section 404 ( b ) of the Sarbanes - Oxley Act ( 15 U.S.C. 7262 ( b ) ) by the registered public accounting firm that prepared or issued its audit report . □ NO ☑ Indicate by check mark whether the Registrant is a shell company ( as defined in Rule 12b - 2 of the Exchange Act ) . YES As of June 30 , 2020 , the last business day of the Registrant's most recently completed second fiscal quarter , the aggregate market value of the voting and non - voting common equity held by non - affiliates of the Registrant , based on the last reported sale price of the shares of common stock on The Nasdaq Global Market was $ 36,630,063 . As of March 19 , 2021 , the Registrant has 90,210,557 shares of Common Stock , $ 0.001 par value per share , outstanding . Portions of the Registrant's definitive proxy statement for its 2021 Annual Meeting of Stockholders , which the Registrant intends to file pursuant to Regulation 14A with the Securities and Exchange Commission not later than 120 days after the end of the Registrant's fiscal year ended December 31 , 2020 , are incorporated by reference into Part III of this Annual Report on Form 10 - K .