Mean Girls is a 2004 American teen comedy film directed by Mark Waters, written by Tina Fey, and starring Lindsay Lohan, Rachel McAdams, Lacey Chabert, Amanda Seyfried (in her film acting debut), Tim Meadows, Ana Gasteyer, Amy Poehler and Fey. The supporting cast includes Lizzy Caplan, Jonathan Bennett, Daniel Franzese, and Neil Flynn. The plot centers on Cady Heron, a naïve teenage girl played by Lohan, who navigates her way through the social hierarchy of a modern American high school after years of homeschooling by her parents as they conducted research in Africa.

The screenplay is based on Rosalind Wiseman's 2002 book Queen Bees and Wannabes, which describes female high school social cliques, school bullying and the damaging effect they can have on teenagers. Fey also drew from her own experience at Upper Darby High School, in Upper Darby Township, Pennsylvania, as an inspiration for some of the film's concepts.Saturday Night Live creator Lorne Michaels produced the film. Fey was a long-term cast member and writer for Saturday Night Live. Filming took place from September to November 2003. Although set in the Chicago suburb of Evanston, Illinois, the film was mostly shot in Toronto, Canada. It marks Lohan's second collaboration with director Waters; the first was Freaky Friday, released in 2003.

Released on April 30, 2004, Mean Girls grossed $130.1 million worldwide and has become an enduring classic. Critics praised Waters' direction, Fey's screenplay, its humor and the performances; especially lauded was Lohan's acting, which earned several accolades including three Teen Choice Awards and two MTV Movie Awards, and in 2021, was listed as the eleventh-best performance of the 21st century by The New Yorker.

A made-for-TV sequel Mean Girls 2, premiered on ABC Family in January 2011. Mean Girls also spawned various adaptations, including a stage musical, which premiered on Broadway in March 2018, with a film adaptation that was released on January 12, 2024.