Phoenix is best known for his work in "Parenthood," "Gladiator" and "Walk the Line.
"I'm Still Here" is his brother-in-law Casey Affleck's directorial debut.
The film follows Phoenix as he announces his retirement from a successful film career in the fall of 2008 and sets off to reinvent himself as a hip-hop musician.
"Sometimes funny, sometimes shocking, and always riveting, the film is a portrait of an artist at a crossroads," the movie's distributors said in a news release Wednesday. "Defying expectations, it deftly explores notions of courage and creative reinvention, as well as the ramifications of a life spent in the public eye."
Magnolia is to release "I'm Still Here" in U.S. theaters Sept. 10.
The younger brother of actor-director Ben Affleck, Casey Affleck has appeared in "Good Will Hunting," "Gone Baby Gone" and "The Assassination of Jesse James by the Coward Robert Ford."