The Hot Right Now singer, 23, will play Christian Grey's sister in the big-screen adaptation of EL James's erotic novel.
Ora tweeted the news to her fans, writing: "It's official! I've been cast in Fifty Shades Of Grey as Christian's sister Mia."
British actor Jamie Dornan is set to play Grey in the movie, which is being directed by filmmaker and artist Sam Taylor-Johnson.
He signed up to star as the billionaire, who has a passionate relationship with student Anastasia Steele (Dakota Johnson), after Charlie Hunnam quit the role.
Ora, who was born in Kosovo, educated at London's Sylvia Young Theatre School and appeared in 2004 film Spivs, was at the British Fashion Awards when she tweeted the news about her first major film role.
The movie, which began shooting in Vancouver, Canada on Sunday, is set for release on Valentine's Day weekend, 2015.