Depp was originally cast in Black Mass last February, but dropped out in May after he was reportedly asked to take a $US10 million ($A11.21 million) pay cut due to budget constraints.
Last month, it was reported the actor was back in talks and now he has officially signed on to play the Boston gangster, Deadline.com reports.
Bulger spent almost two decades evading capture after he was placed on the Federal Bureau of Investigation's (FBI) Most Wanted List.
Director Barry Levinson was initially attached to direct the film, but Out of the Furnace filmmaker Scott Cooper has taken over the project.
In addition to Depp's casting, The Dark Knight Rises actor Tom Hardy is reportedly in talks to star as former FBI agent John Connolly, a childhood friend of Bulger who served time in prison for racketeering and obstruction of justice charges stemming from his relationship with him.
Production is expected to begin in May.
Black Mass isn't the only film based on Bulger's life story in development - Matt Damon is also set to star as the crime lord in a project directed by his pal Ben Affleck, while Twilight actor Peter Facinelli is working on a TV adaptation of Edward MacKenzie and Phyllis Karas' book Street Soldier: My Life as an Enforcer For Whitey Bulger and the Boston Irish Mob.
