Hamill, who earlier said he won't be voicing The Joker after doing it for 18 years in various DC animated TV shows, movies, and video games, agreed to voice the villain in the film which was announced at the comic-Con, reported Entertainment Weekly.
Written by Alan Moore and illustrated by Brian Bolland, "The Killing Joke" comic is famous for dealing with a cruel fate to Batgirl Barbara Gordon.
The 63-year-old actor, best known for portraying Luke Skywalker in the original "Star Wars" trilogy, had said that an animated take on "The Killing Joke" might be one of the projects that might make him return to the role.
The actor will be seen returning to his famous character, Luke, in "Star Wars: The Force Awakens", the seventh instalment in the film series.