The 31-year-old, who played Laurent in the vampire romance, will play the mixed race mutant Darwin in Matthew Vaughn's forthcoming sci-fi blockbuster.
Kenyan-born Gathegi played the Haitian cabbie in Crank and subsequently played Cheese in the thriller Gone Baby Gone although he is probably best known for the role of Dr Jeffrey Cole (aka "Big Love") in the TV series House,
In the comic, Darwin kicks off as the half-black/half-latino Armando Muñoz, a mutant who is able to adapt to any situation, from growing gills if stuck underwater to sporting fire-proof skin when things hot up.
The cast already includes James McAvoy as the young Professor X, Michael Fassbender as Erik Lensherr/Magneto, Kevin Bacon as Sebastian Shaw, Alice Eve as Emma Frost, Nicholas Hoult as Hank McCoy, also known as Beast, and Jennifer Lawrence as Mystique.
There's more... Caleb Landry Jones is Banshee, Lucas Till is Havok and Josh Hutcherson is rumoured to be a contender for the role of Cyclops.