The Walk the Line star has been attached to the film for quite a while, but he has consistently played coy about the gossip, telling Collider in April he simply didn't know if he had the part. It will be distributed worldwide by Warner Bros.
According to The Hollywood Reporter, the standalone film was picked up by Warner Bros. and will begin filming in NY this September with Phoenix playing Batman's arch-nemesis.
Phillips ("The Hangover" franchise) is on board to co-write and direct the movie.
Warner Bros. Pictures has officially announced that Joaquin Phoenix will star in an origin story of one of DC's most notorious super-villains, the Joker. Production on the project is set to get underway this September in New York City with a budget of $55 million.
Phoenix, 43, has previously said he wasn't against taking on the role of the Joker after reports named him as the next actor to personify the sinister comic character.
"It's kind of built into the source material in some ways".
The standalone "Joker" feature will fall under a yet-to-be-announced banner of DC Comics, which gives the studio the opportunity to expand the canon of the iconic comic-book characters with unique story angles that aren't included in its current cinematic universe.