Mean Girls is a 2004 American teen comedy film directed by Mark Waters and written by Tina Fey. It stars Lindsay Lohan, Rachel McAdams, Tim Meadows, Ana Gasteyer, Amy Poehler, and Fey. The film follows Cady Heron (Lohan), a naïve teenager who transfers to an American high school after years of homeschooling in Africa. Cady quickly befriends outcasts Janis and Damian (Lizzy Caplan and Daniel Franzese), with the trio forming a plan to exact revenge on Regina George (McAdams), the leader of an envied clique known as "the Plastics".

Fey conceived the idea for Mean Girls after reading the self-help book Queen Bees and Wannabes. The book describes female high school social cliques, school bullying, and the resulting damaging effect 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 served as a producer; Fey was a long-term cast member and writer for Saturday Night Live. Principal photography took place from September to November 2003. Although set in the Chicago suburb of Evanston, Illinois, filming took place primarily in Toronto, Ontario.

Mean Girls premiered at the Cinerama Dome in Los Angeles on April 19, 2004, and was theatrically released in the United States on April 30, by Paramount Pictures. The film grossed over $130 million worldwide and received generally positive reviews from critics, who praised Waters's 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-television 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 released in January 2024.