Sources told entertainment portal Deadline that the "Succession" star has joined the third installment in the Marvel Studios' upcoming feature film.
It is unknown who Macfadyen will be playing, the sources said.
Also part of the cast are "The Crown" star Emma Corrin and Hugh Jackman, who is reprising his Wolverine role in "Deadpool 3".
Shawn Levy is on board to direct the movie with Paul Wernick and Rhett Reese returning to pen the script. Wendy Molyneux and Lizzie Molyneux-Logelin penned a previous draft.
Marvel Studios president Kevin Feige will produce the movie along with Reynolds and Levy.
It marks the first "Deadpool" collaboration between Marvel Studios and Reynolds & 'Team Deadpool'.