In the Mouth of Madness is a 1994 American supernatural horror film directed and scored by John Carpenter and written by Michael De Luca. It stars Sam Neill, Julie Carmen, Jürgen Prochnow, David Warner and Charlton Heston. Neill stars as John Trent, an insurance investigator who visits a small town while looking into the disappearance of a successful author of horror novels, and begins to question his sanity as the lines between reality and fiction seem to blur. Informally, the film is the third installment in what Carpenter refers to as his "Apocalypse Trilogy", preceded by The Thing (1982) and Prince of Darkness (1987).

In the Mouth of Madness pays tribute to the works of author H. P. Lovecraft in its exploration of insanity, and its title is derived from the Lovecraft novella At the Mountains of Madness. Distributed by New Line Cinema, In the Mouth of Madness received mixed reviews upon release.