The distributors insist it is not a mockumentary.
Independent movie distributors Magnolia Pictures said they had acquired world rights to "I'm Still Here", describing it as "a portrait of an artist at a crossroads."
The movie, directed by Phoenix's brother-in-law, actor Casey Affleck, will open on September 10.
Phoenix's surprise retirement announcement in 2008, followed by a bizarre TV appearance as a mumbling, shaggy-haired guest on "The Late Show with David Letterman", had industry watchers wondering if his new act was a hoax.
Magnolia Pictures spokesman Matt Cowal said "I'm Still Here" is "not a mockumentary by any means. I think the film speaks for itself".
Cowal said Phoenix's participation in promotional events for the release was as yet "undetermined".
Phoenix, 35, was nominated for an Oscar for playing country singer Johnny Cash in the 2005 biopic "Walk the Line".
His last movie was 2008 drama "Two Lovers" opposite Gwyneth Paltrow.
Phoenix said he was quitting acting because he was bored and that hip-hop had always been a passion.
Last year, he said he was working on a mostly self-produced album with help from rapper Sean "Diddy" Combs.
No album has yet been released.