The Ninth Configuration is a 1980 American psychological drama film written, produced, and directed by William Peter Blatty, in his directorial debut. It is the second installment in Blatty's "Trilogy of Faith" after The Exorcist (1973), and followed by The Exorcist III (1990). The film is based on Blatty's 1978 novel The Ninth Configuration, which was itself a reworking of his 1966 novel Twinkle, Twinkle, "Killer" Kane! The initial 1966 publication of the novel featured an exclamation mark at the end of the title, while all subsequent publications saw it removed.

William Peter Blatty was an American writer, director and producer. He is best known for his 1971 novel, The Exorcist, and for his 1973 screenplay for the film adaptation of the same name. Blatty won an Academy Award for Best Adapted Screenplay for The Exorcist, and was nominated for Best Picture as its producer. The film also earned Blatty a Golden Globe Award for Best Motion Picture – Drama as producer.