The production company released a new photo for the film Spencer on Thursday featuring Stewart, 30, as Diana, born Diana Spencer.
Neon previously shared a photo of Stewart wearing a red coat and black hat as Diana.
Poldark actor Jack Farthing has been cast as Charles. Timothy Spall, Sean Harris and Sally Hawkins will also star in the new film.
Spencer is written by Steven Knight and directed by Pablo Larraín. The film takes place over a holiday weekend in the early 1990s when Diana decided to leave her marriage to Prince Charles.
"Spencer is a dive inside an emotional imagining of who Diana was at a pivotal turning point in her life," Stewart previously said in a statement.
Diana and Charles were married from 1981 to 1996 and have two sons, Prince William and Prince Harry. Diana died in a car crash at age 36 in August 1997.
Stewart said on Jimmy Kimmel Live! in November that she feels "protective" of Diana after playing her in the new film.
Spencer is expected to open in theaters in the fall.