X2 (also marketed as X2: X-Men United, and internationally as X-Men 2) is a 2003 American superhero film directed by Bryan Singer and written by Michael Dougherty, Dan Harris and David Hayter, from a story by Singer, Hayter and Zak Penn. The film is based on the X-Men superhero team appearing in Marvel Comics. It is the sequel to X-Men (2000), as well as the second installment in the X-Men film series, and features an ensemble cast including Patrick Stewart, Hugh Jackman, Ian McKellen, Halle Berry, Famke Janssen, James Marsden, Rebecca Romijn-Stamos, Brian Cox, Alan Cumming, Bruce Davison, Shawn Ashmore, Aaron Stanford, Kelly Hu, and Anna Paquin. The plot, inspired by the graphic novel God Loves, Man Kills, concerns the genocidal Colonel William Stryker leading an assault on Professor Xavier's school to build his own version of Xavier's mutant-tracking computer, Cerebro, in order to destroy every mutant on Earth and to save the human race from them, forcing the X-Men to team up with the Brotherhood of Mutants to stop Stryker and save the mutant race.

Development on the sequel began shortly after the first film was released on July 14, 2000, by 20th Century Fox. David Hayter and Zak Penn wrote separate scripts, combining what they felt to be the best elements of both scripts into one screenplay. Michael Dougherty and Dan Harris were eventually hired to rewrite the work, and changed the characterizations of Beast, Angel, and Lady Deathstrike. Sentinels and the Danger Room were set to appear before being deleted because of budget concerns from Fox. The film's premise was influenced by the Marvel Comics storylines Return to Weapon X and God Loves, Man Kills. Filming began in June 2002 and ended that November, mostly taking place at Vancouver Film Studios, the largest North American production facility outside of Los Angeles. Production designer Guy Hendrix Dyas adapted similar designs by John Myhre from the previous film.

X2 was released in the United States on May 2, 2003, by 20th Century Fox, and received positive reviews for its storyline, musical score, action sequences, and performances. The film grossed $407 million worldwide, making it the ninth-highest-grossing film of 2003, and received eight Saturn Awards nominations. A sequel, X-Men: The Last Stand, was released on May 26, 2006.