William J. Monahan is an American screenwriter and novelist. His second produced screenplay was The Departed, a film that earned him a Writers Guild of America Award and Academy Award for Best Adapted Screenplay.

The Departed is a 2006 American epic crime thriller film film directed by Martin Scorsese and written by William Monahan. It is both a remake of the 2002 Hong Kong film Infernal Affairs and also loosely based on the real-life Boston Winter Hill Gang; the character Colin Sullivan is based on the corrupt FBI agent John Connolly, while the character Frank Costello is based on Irish-American gangster and crime boss Whitey Bulger. The film stars Leonardo DiCaprio, Matt Damon, Jack Nicholson, and Mark Wahlberg, with Martin Sheen, Ray Winstone, Vera Farmiga, Alec Baldwin, Anthony Anderson and James Badge Dale in supporting roles.