Phoenix, whose name had been doing the rounds to star as Batman's arch-nemesis for a long time, recently finalised the deal.
According to The Hollywood Reporter, the shooting is set to begin in September in New York.
Todd Phillips is directing the film, which the studio describes as being an "exploration of a man disregarded by society (that) is not only a gritty character study, but also a broader cautionary tale".
The standalone is meant to be darker and more experimental in tone and content and is being described as a crime drama.
The project's budget is in the USD 55 million range, significantly lower than what studios generally allot for a film belonging to the superhero/villain genre.
Phillips co-wrote the script with Scott Silver.
The release date is yet to be announced by Warners.
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