De Niro is in negotiations to join the project as a talk-show host who plays a role in Phoenix becoming the Joker, Deadline reported.
This would be De Niro's first role in a comic book film.
Joker is being directed by Todd Phillips (The Hangover, War Dogs) based on a script he co-wrote with Scott Silver. The project, set for release on Oct. 4, 2019, is described as being a gritty character study and a cautionary tale.
The film will reportedly be set in Gotham City in the early 1980s, noted The Hollywood Reporter.
Zazie Beetz, who recently starred in Fox's Deadpool 2, is also in negotiations to join Joker as a single mother who catches the eye of Phoenix.
Joker will take place outside of DC's extended film universe and would help launch a new DC Comics banner at Warner Bros. that will allow the studio to tell unique comic book stories using different actors in the title roles. Production is set to begin in September in New York.