The Ninth Configuration (also known as Twinkle, Twinkle, "Killer" Kane) 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.

The first half of the film has the predominant tone and style of a comic farce. In the second half, the film becomes darker as it delves deeper into its central issues of human suffering, sacrifice and faith. The film also frequently blurs the line between the sane and insane. The film received positive reviews from critics, and was recognized with the Golden Globe Award for Best Screenplay and two other nominations at the 38th Golden Globe Awards.