The 32-year-old actress will play Angel Dust in the Ryan Reynolds-led Marvel movie, according to The Hollywood Reporter.
In the Marvel comics, Angel Dust was part of an underground mutant group known as The Morlocks.
Tim Miller is directing the film starring Reynolds as the character he played in 'X-Men Origins: Wolverine', with production due to start in Vancouver in March.
Producer Simon Kinberg has already revealed the character will tie in with the other X-Men films, saying: "There's definitely a sort of overall plan that we've all been talking about for the X-Men universe now and Deadpool obviously fits into that."
'Deadpool' is slated for release in February 2016.