50 Cent has joined the cast of 'Southpaw'.

The 'Candy Shop' rapper will star alongside Jake Gyllenhaal, Forest Whitaker, Lupita Nyong'o and Rachel McAdams in the boxing movie, in which he will play a promoter who is part of Gyllenhaal's central character's entourage.

50 has already begun filming under director Antoine Fuqua in Pittsburgh, the New York Post's Page Six column reports.

The film centres on the welterweight boxer as he struggles to improve his career as he battles with substance abuse.

Nyong'o's social worker character will care for the sportsman's daughter and her profession will eventually lead her to become involved in the life of the fallen boxer.

Fuqua recently spoke about his passion for the movie, and how delighted it was when it changed studios and he was able to sign on to direct.

He said: '''Southpaw' was with DreamWorks, that's when I got the job.

''I have a great passion for that project, and I met with [Steven] Spielberg about it, and I loved it. The timing of that didn't work up. And I spoke with Harvey Weinstein, and he loved it from the beginning. So I said, let's do it. So he went over and talked to them, and Harvey has it now.''