Variety reported Wednesday that Williams, 40, will portray Katharine Parr in the upcoming psychological horror film.
The Hollywood Reporter confirmed Williams' casting.
Firebrand is directed by Karim Aïnouz and marks the director's English-language debut. Aïnouz's most recent film, Mariner of the Mountains, will have a special screening at the Cannes Film Festival this month.
Firebrand is written by Henrietta and Jessica Ashworth (Killing Eve) and produced by Gabrielle Tana (The Dig).
"I could not be more excited to be bringing the undiscovered story of Katharine Parr to the screen, a ferociously brilliant woman who I am inspired by deeply and has been largely invisible, or certainly underrepresented in English history," Aïnouz said. "Much is known about Henry VIII's tyrannical reign, and those who perished and suffered at his hands, but my focus here is on a woman who not only managed to survive, but also, to thrive."
"This is a reimagining of a 'period' film, a psychological horror film set in the Tudor court; a story of intrigue, agency and survival. Having Michelle Williams portray this remarkable woman, an actress of immeasurable talent and passion, is a dream come true," he added.
Williams is known for the films Brokeback Mountain, My Week with Marilyn, Manchester by the Sea and The Greatest Showman. She played Jen Lindley on Dawson's Creek.