X-Men: First Class is a 2011 superhero film based on the X-Men characters appearing in Marvel Comics. It is the fourth mainline installment in the X-Men film series and the fifth overall. It was directed by Matthew Vaughn and produced by Bryan Singer, and stars James McAvoy, Michael Fassbender, Rose Byrne, Jennifer Lawrence, January Jones, Oliver Platt, and Kevin Bacon. At the time of its release, it was intended to be a franchise reboot and contradicted the events of previous films; however, the follow-up film X-Men: Days of Future Past (2014) retconned First Class into a prequel to X-Men (2000). First Class is set primarily in 1962 during the Cuban Missile Crisis, and focuses on the relationship between Charles Xavier and Erik Lehnsherr / Magneto, and the origin of their groups—the X-Men and the Brotherhood of Mutants, respectively, as they deal with the Hellfire Club led by Sebastian Shaw, a mutant supremacist bent on starting a nuclear war.

Producer Lauren Shuler Donner first thought of a prequel based on the young X-Men during the production of X2; producer Simon Kinberg later suggested to 20th Century Fox an adaptation of the comic series X-Men: First Class, although the film does not follow the comic closely. Singer, who had directed both X-Men and X2, became involved with the project in 2009, but he could only produce and co-write First Class due to his work on other projects. Vaughn became the director and also wrote the final script with his writing partner Jane Goldman. Principal photography began in August 2010 and concluded in December, with additional filming completed in April 2011. Locations included Oxford, the Mojave Desert and Georgia, with soundstage work done in both Pinewood Studios and the 20th Century Fox stages in Los Angeles. The depiction of the 1960s drew inspiration from the James Bond films of the period.

First Class premiered in Ziegfeld Theatre on May 25, 2011, and was released in the United States on June 3. It was a box office success, grossing $353 million worldwide, and received positive reviews from critics and audiences, who praised its acting, screenplay, direction, action sequences, visual effects, and musical score. The film's success re-popularized the X-Men film franchise with various installments following, including a number of sequels focusing on younger iterations of the X-Men characters, with X-Men: Days of Future Past (2014), X-Men: Apocalypse (2016), and Dark Phoenix (2019).

In 1944, at the Auschwitz concentration camp, Nazi officer Klaus Schmidt witnesses a young Erik Lehnsherr bending a metal gate with his mind upon being separated from his parents. Schmidt brings Lehnsherr into his office and tells him to move a coin on his desk. When Lehnsherr cannot do it, Schmidt kills his mother. Distraught, Lehnsherr's magnetic power manifests, destroying the room. Meanwhile, at a mansion in Westchester County, New York, young telepath Charles Xavier meets Raven, a scaly blue-skinned shapeshifter. He invites her to live with him as his sister.

In 1962, Lehnsherr tracks Schmidt while Xavier earns his doctorate from the University of Oxford. In Las Vegas, CIA officer Moira MacTaggert follows US Army Colonel Hendry into the Hellfire Club, where Hendry meets with Schmidt (now called Sebastian Shaw), mutant telepath Emma Frost, cyclone-producing Riptide, and teleporter Azazel. Threatened by Shaw and teleported to the Joint War Room, Hendry advocates deploying nuclear missiles in Turkey. Shaw, an energy-absorbing mutant whose powers have kept him young, later kills Hendry.

MacTaggert, seeking Xavier's advice on mutation, takes him and Raven to the CIA, where they convince Director McCone that mutants exist and that Shaw is a threat. Another CIA officer sponsors the mutants and invites them to the secret "Division X" facility. MacTaggert and Xavier find Shaw as Lehnsherr attacks him, and Xavier rescues Lehnsherr from drowning before Shaw escapes. Xavier brings Lehnsherr to Division X, where they meet Hank McCoy, a mutant scientist with prehensile feet. Xavier uses McCoy's mutant-locating device, Cerebro, to seek and recruit other mutants; Angel Salvadore, Armando Muñoz, Alex Summers, and Sean Cassidy.

Xavier, Lehnsherr, and MacTaggert lead a CIA mission to the Soviet Union to capture Frost and discover Shaw intends to start World War III, triggering mutant ascendency. Azazel, Riptide, and Shaw attack Division X, killing everyone but the mutants whom Shaw invites to join him. Angel accepts, but when Alex and Armando retaliate, Shaw kills Armando. In Moscow, Shaw compels the generals to have the USSR install missiles in Cuba. Wearing a helmet that blocks telepathy, Shaw follows the Soviet fleet in a submarine to ensure the missiles break a U.S. blockade. In the meantime, Xavier takes the remaining recruits back to his mansion, where they focus on harnessing their abilities. McCoy believes Raven's DNA may provide a "cure" for their appearance and manages to get a cure ready, but Raven, after being persuaded by Lehnsherr, decides she does not want to hide her identity and refuses the cure. McCoy uses the cure on himself, but it backfires, giving him blue fur and a leonine appearance.

With McCoy piloting, the mutants and MacTaggert fly to the blockade line. Xavier uses telepathy to make a Soviet sailor destroy the missile ship, while Lehnsherr uses his magnetic powers to lift Shaw's submarine onto land. Lehnsherr grabs Shaw's helmet in battle, allowing Xavier to immobilize him. However, Lehnsherr reveals that he shares Shaw's exclusivist view of mutants but wants revenge for his mother's death. Lehnsherr freezes time, immobilising Shaw, before killing him by throwing the Nazi decorated coin through his head.