Under the direction of Todd Phillips (The Hangover franchise), the new film centers around the iconic arch nemesis and is an original, standalone story not seen before on the big screen.
Phoenix has previously discussed Leto's work as the Joker and revealed he would take the character in a different direction than his predecessors.
The budget is also significantly lower than most "superhero" films at around $55million.
"I don't know...it could be an interesting character, I don't know", Phoenix told Fandango in April, adding that he wouldn't be opposed to comic book movies if the conditions were right.
"I see [a superhero movie] as any other movie", Phoenix continued.
The Joker is known for both his wit and creative cruelty, so it could be Joaquin Phoenix's turn to show off his villainous side playing the well-known comic book character.
Warner Bros. has also not yet announced a release date, but THR says there's a chance the film could be ready for release as soon as late 2019.