The Boondock Saints is a 1999 vigilante action thriller film written and directed by Troy Duffy in his feature directorial debut. Starring Willem Dafoe, Sean Patrick Flanery, Norman Reedus, David Della Rocco, and Billy Connolly, the film follows Irish fraternal twin brothers Connor and Murphy MacManus (Flanery and Reedus), who become vigilantes after killing two members of the Russian mafia in self defense. After both experience an epiphany, the twins, together with their best friend "Funny Man" Rocco (Rocco), set out on a mission to rid Boston of the criminal underworld in the name of God, all the while being pursued by FBI Special Agent Paul Smecker (Dafoe).

Duffy, who had never written a screenplay before, said he was inspired by personal experience while living with his brother Taylor in Los Angeles. Initially regarded as one of the hottest scripts in Hollywood, the film had a troubled production. Miramax Films dropped the project in 1997 before Franchise Pictures acquired the rights the following year. Principal photography began in Boston and Toronto on August 10, 1998, and concluded on September 26.

The theatrical release of The Boondock Saints was significantly affected by the Columbine High School massacre, which had taken place just two weeks before test screenings. Amidst concerns that the film would inspire copycat crimes, it was given a limited release in only five theaters across the United States on January 21, 2000. Consequently, the film was a box office failure and received negative reviews from critics, with criticism aimed at its perceived glorification of vigilante justice and violence. Despite this, The Boondock Saints became a cult classic through word of mouth and its home video release, ultimately grossing $50 million in sales.

A successful 2006 theatrical rerelease led to a sequel, The Boondock Saints II: All Saints Day (2009), with Flanery, Reedus, Connolly, and Rocco reprising their roles, and Dafoe making an uncredited cameo appearance. Overnight (2003), a documentary about the making of the film, was also released. A third film is currently in development, with Flanery and Reedus expected to return.