The GI Joe star will appear opposite mixed martial arts fighter Gina Carano in the film, which centres on a girl from the wrong side of the tracks who is recruited into a Blackwater-style security contractor.
According to The Playlist, Tatum will play a military specialist assigned to bring in Carano's character when she goes rogue.
Ewan McGregor, Michael Douglas, Dennis Quaid and Michael Fassbender were last week revealed to be joining the cast. Antonio Banderas is also in talks to appear as the head of a Black-ops European unit.
Knockout begins shooting next month in the US, Ireland and Turkey.