UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, DC 20549

FORM 6-K

REPORT OF FOREIGN PRIVATE ISSUER

PURSUANT TO RULE 13a-16 OR 15d-16 UNDER

THE SECURITIES EXCHANGE ACT OF 1934

For the month of May 2025

Commission File Number: 001-32371

SINOVAC BIOTECH LTD.

No. 39 Shangdi Xi Rd, Haidian District

Beijing 100085, People’s Republic of China

(Address of principal executive offices)

Indicate by check mark whether the registrant files or will file annual reports under cover of Form 20-F or Form 40-F:

Form 20-F x               Form 40-F ¨

CONTENT

Departure of Director

On May 16, 2025, the board of directors (the "Board") of Sinovac Biotech Ltd. received a resignation notice from Mr. Pengfei Li, a member of the Board, with immediate effect. Mr. Li’s resignation will not affect the composition of any committee of the Board as he had not been serving on any committee.

SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

Date: May 19, 2025