Item 4.01. Changes in Registrant’s Certifying Accountant. (a) Dismissal of Independent Registered Public Accounting Firm The Audit Committee (the “ Committee”) of the Board of Directors of Applied Optoelectronics, Inc. (the “ Company”) recently completed a competitive selection process, inclusive of the Company’s current independent registered public accounting firm, Grant Thornton LLP (“ Grant Thornton”), to determine the Company’s independent registered public accounting firm for the fiscal year ending December 31, 2026. Following the review and evaluation of the proposals of the participating firms, on May 4, 2026, the Committee approved the dismissal of Grant Thornton as the Company’s independent registered public accounting firm. Grant Thronton was previously engaged to perform a review of the Company’s consolidated financial statements for the quarter ending March 31, 2026. The dismissal of Grant Thornton does not affect its engagement for the quarter ending March 31, 2026. Grant Thornton’s reports on the Company’s consolidated financial statements as of and for the fiscal years ended December 31, 2025 and 2024 did not contain any adverse opinion or disclaimer of opinion, nor were they qualified or modified as to uncertainty, audit scope, or accounting principles. During the fiscal years ended December 31, 2025 and 2024 and the subsequent interim period through May 4, 2026, there were no disagreements (as defined in Item 304(a)(1)(iv) of Regulation S-K and the related instructions) with Grant Thornton on any matter of accounting principles or practices, financial statement disclosure, or auditing scope or procedures, which disagreements, if not resolved to Grant Thornton’s satisfaction, would have caused Grant Thornton to make reference thereto in their reports. There were no “reportable events” (as that term is defined in Item 304(a)(1)(v) of Regulation S-K) during either of the years ended December 31, 2024 and 2025, and the subsequent interim period through May 4, 2026, except for the material weakness in the Company’s operation of controls over its review of technical accounting analysis, as first disclosed in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2024, which material weakness was remediated as of December 31, 2025. The Company provided Grant Thornton with a copy of this Current Report on Form 8-K prior to its filing with the United States Securities and Exchange Commission (the “ SEC”) and requested that Grant Thornton furnish a letter addressed to the SEC stating whether or not it agrees with the statements made herein, as specified by Item 304(a)(3) of Regulation S-K. A copy of Grant Thornton’s letter, dated May 8, 2026, is filed as Exhibit 16.1 to this Current Report on Form 8-K. (b) Appointment of New Independent Registered Public Accounting Firm As a result of the process noted above and following the review and evaluation of proposals from all participating firms, on May 4, 2026, the Committee approved the appointment of PricewaterhouseCoopers LLP (“ PwC”) as the Company’s independent registered public accounting firm for the fiscal year ending December 31, 2026. During the fiscal years ended December 31, 2025 and 2024 and the subsequent interim period through May 4, 2026, neither the Company nor anyone on its behalf consulted with PwC regarding the application of accounting principles to a specified transaction, either completed or proposed, the type of audit opinion that would have been rendered on the Company’s consolidated financial statements, or any matter that was the subject of a “disagreement” (as that term is defined in Item 304(a)(1)(iv) of Regulation S-K and the related instructions) or any “reportable event” (as that term is defined in Item 304(a)(1)(v) of Regulation S-K) or any other matters set forth in Item 304(a)(2)(i) or (ii) of Regulation S-K. 2

Item 9.01. Financial Statements and Exhibits. (d) Exhibits.