One Flew Over the Cuckoo's Nest is a 1975 American psychological comedy-drama film directed by Miloš Forman, based on the novel by Ken Kesey. The film stars Jack Nicholson as a new patient at a mental institution and Louise Fletcher as the domineering head nurse. Will Sampson, Danny DeVito, Sydney Lassick, William Redfield, Christopher Lloyd and Brad Dourif play supporting roles, with the latter two making their feature film debuts.

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 psychiatric hospital, which is also the novel's setting. The hospital is still in operation, though the original buildings in the film have been demolished. The film was released on November 19, 1975.

One Flew Over the Cuckoo's Nest received critical acclaim, and is considered by critics and audiences to be one of the greatest films ever made. It is the second of three films to win all five major Academy Awards (Best Picture, Best Actor, Best Actress, Director, and Screenplay) following It Happened One Night (1934), and preceding 1991's 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. Additionally, One Flew Over the Cuckoo's Nest was ranked No. 33 on the American Film Institute's updated 100 Years... 100 Movies list in 2007.