One Flew Over the Cuckoo's Nest is a 1975 American psychological drama film directed by Miloš Forman, based on the novel One Flew Over the Cuckoo's Nest by Ken Kesey. The film stars Jack Nicholson as a new patient at a mental institution, alongside Louise Fletcher as an austere nurse. The supporting cast is Will Sampson, Danny DeVito, Sydney Lassick, William Redfield, and the film debuts of Christopher Lloyd and Brad Dourif.

Originally announced in 1962 with Kirk Douglas starring, the film took 13 years to develop. Filming finally began in January 1975 and lasted three months, on location in Salem, Oregon and the surrounding area, and in Depoe Bay on the north Oregon coast. The producers shot the film in the Oregon State Hospital, an actual mental hospital, which is also the setting of the novel. The hospital is in operation, though the original buildings in the film have been demolished. Forman, a refugee who fled Czechoslovakia after the end of the Prague Spring, drew upon his experiences living under Communist rule as inspiration for the oppressive environment of the mental hospital. The film was released on November 19, 1975. Considered by many to be one of the greatest films ever made, One Flew Over the Cuckoo's Nest is No. 33 on the American Film Institute's 100 Years... 100 Movies list.

The film is the second to win all five major Academy Awards (Best Picture, Best Actor, Best Actress, Director, and Screenplay) following It Happened One Night in 1934, an accomplishment not repeated until 1991 with The Silence of the Lambs. It won numerous Golden Globe and BAFTA Awards. In 1993, the film was deemed "culturally, historically, or aesthetically significant" by the United States Library of Congress, and selected for preservation in the National Film Registry.