Infernal Affairs is a 2002 Hong Kong action thriller film directed by Andrew Lau and Alan Mak from a screenplay written by Mak and Felix Chong. The film stars Andy Lau, Tony Leung, Anthony Wong, Eric Tsang, Sammi Cheng and Kelly Chen. The film follows an undercover Hong Kong Police Force officer who infiltrates a triad and another police officer who is secretly a spy for the same triad. It is the first in the Infernal Affairs series and is followed by Infernal Affairs II and Infernal Affairs III (both 2003).

At the 22nd Hong Kong Film Awards, Infernal Affairs won seven out of the sixteen awards it was nominated for—including Best Film, Best Director, Best Actor (Leung), and Best Supporting Actor (Wong). It also won in those categories at the 40th Golden Horse Awards and 8th Golden Bauhinia Awards. The film was selected as Hong Kong's entry for the Best Foreign Language Film at the 76th Academy Awards but was not nominated. Miramax Films acquired the United States distribution rights and gave it a limited US theatrical release in 2004. Martin Scorsese remade the film in 2006 as The Departed, which won the Academy Award for Best Picture as well as Academy Award for Best Director, Scorsese's first and only Oscar in his career, and Best Adapted Screenplay.

A 4K remaster of the Infernal Affairs trilogy was released on 12 December 2022, to celebrate the 20th anniversary of Infernal Affairs.