The production company shared a first photo Wednesday of Stewart, 30, in the new film Spencer, directed by Pablo Larraín (Jackie).
The picture shows Stewart wearing a black hat with veil, black pussybow blouse and a red coat. She sports Diana's signature short blonde hairstyle.
"Kristen Stewart is Diana, Princess of Wales, in Pablo Larraín's Spencer," the post reads.
Variety said principal photography is underway on Spencer. Filming will take place in Germany and the United Kingdom, with the movie expected to premiere in fall 2021.
"Spencer is a dive inside an emotional imagining of who Diana was at a pivotal turning point in her life," Stewart said in a statement. "It is a physical assertion of the sum of her parts, which starts with her given name: Spencer. It is a harrowing effort for her to return to herself, as Diana strives to hold onto what the name Spencer means to her."
Stewart said on Jimmy Kimmel Live! in November that she feels protective of Diana after playing the late British royal in Spencer.
"It's hard not to feel protective of her. She was so young," Stewart said. "Everyone's perspective is different, and there's no way to get anything right because what is fact in relation to personal experience?"
Diana died in a car crash at age 36 in August 1997. She is portrayed by Emma Corrin in the Netflix series The Crown.