The 41-year-old actress, who featured in 2006's "Mission: Impossible III" as Julia Meade, fiancee to Cruise's Ethan Hunt, took to Instagram to break the news.
"Mission accepted. So thrilled to be reuniting with Tom Cruise for #MI6! Can't wait to show you guys more of Julia's story #missionimpossible," Monaghan posted alongside a selfie.
The film, written and directed by Christopher McQuarrie, marks the actress' third appearance in the franchise.
She also appeared at the end of 2011's "Mission: Impossible - Ghost Protocol".
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The shooting of the film started on April 8 in Paris.
Rebecca Ferguson, Ving Rhames, Simon Pegg, Alec Baldwin and Sean Harris are reprising their roles from previous films while Henry Cavill, Vanessa Kirby and Sian Brooke have come onboard as new characters.
The film is slated to release on July 27, 2018.
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