Underworld is a 2003 action horror film directed by Len Wiseman in his feature film directorial debut, from a screenplay by Danny McBride, based on a story by Kevin Grevioux, Wiseman, and McBride. The film stars Kate Beckinsale, Scott Speedman, Michael Sheen, Shane Brolly, and Bill Nighy. The plot centers on the secret history of vampires and lycans (an abbreviated form of lycanthrope, which means werewolf). The main plot revolves around Selene (Beckinsale), a vampire Death Dealer hunting Lycans. She finds herself attracted to a human, Michael Corvin (Speedman), who is being targeted by the Lycans. After Michael is bitten by a Lycan, Selene must decide whether to do her duty and kill him or go against her clan and save him.

Underworld premiered at the Grauman's Chinese Theatre in Los Angeles, California on September 15, 2003, and was released in the United States on September 19, by Sony Pictures Releasing. The film received generally negative reviews from critics, but a smaller number of reviewers praised elements such as the film's Gothic visuals, Kate Beckinsale's performance, and the extensively worked-out vampire–werewolf mythology that serves as the film's backstory. A surprise hit, the film grossed over $95 million worldwide against a production budget of $22 million and originated a franchise.

The film was followed by Underworld: Evolution in 2006.