The singer is set to play Mia, the sister of lead character Christian Grey, played by Jamie Dornan.
Rita confirmed her casting ahead of the British Fashion Awards on Monday night.
She tweeted: "It's official! I've been cast in @FiftyShades of Grey as Christian's sister Mia. #FiftyShades."
Rita previously had a small role in 'Fast & Furious 6' - a part which she was believed to have beat Rihanna to.
Shooting is already underway on the film, with Jamie pictured on set with Anastasia Steele actress Dakota Johnson earlier this week.
'50 Shades Of Grey' will be released on Valentine's Day weekend in 2015.