Rita will play the role of play Mia, the adopted younger sister of Jamie Dornan's male lead, Christian Grey, in the movie version of E. L. James' bestselling novel.
The 'Shine Ya Light' singer made the announcement as she attended the British Fashion Awards on Monday night (2 December). She tweeted: "It's official! I've been cast in @FiftyShades of Grey as Christian's sister Mia. #FiftyShades."
This is set to be the 23-year-old's biggest movie to date, as she joins Jamie along with Dakota Johnson, Luke Grimes, Eloise Mumford and Jennifer Ehle in the erotic film.
The highly-anticipated Fifty Shades of Grey, expected for release on Valentine's Day in 2015, tells the story of a billionaire (Christian), who embarks on a passionate affair with student Anastasia Steele.
Fans were given a first glimpse of what the characters will look like on the big screen, when Jamie and Dakota graced the front cover of Entertainment Weekly last month.
Rita has appeared on the big screen before, having acted in 2004 film Spivs, as well as making an appearance in US drama 90210 and having a cameo in action movie Fast & Furious 6 earlier this year.
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Watch Rita Ora's music video for 'Radioactive' below: