In the Mouth of Madness (also known as John Carpenter's In the Mouth of Madness) is a 1995 horror film (originally intended for a 1994 release) directed by John Carpenter and written by Michael de Luca, who was at the time in charge of New Line Cinema.

The film is the third installment in what Carpenter calls his "Apocalypse Trilogy". It is preceded by The Thing and Prince of Darkness.

The story follows private investigator John Trent, played by Sam Neill, whose speciality is insurance fraud. He is called in by his client to investigate the disappearance of a phenomenally popular horror novelist named Sutter Cane (Jürgen Prochnow). Cane has supposedly vanished as his most recent novel is nearing its deadline and his publisher (played by Charlton Heston) wants to retrieve his company's property. Trent thinks the whole thing is a publicity stunt for the new novel.

Trent, along with Cane's editor Linda Styles (played by Julie Carmen), eventually tracks the writer down to the remote New England town of Hobbs End which previously was thought to only exist in Cane's stories. There it soon becomes clear that the wall between fantasy and reality is blurring.

This becomes central to the movie's satirical themes, which focus on the relationship between writer and audience in a way that comments ironically upon oft-stated fears that violent entertainment can have a psychological effect on its audience, causing it to lose touch with reality and develop violent behavior. Fans of Cane's are shown rioting in bookstores when they are unable to find his latest novel and by the end of the movie we learn that society itself has collapsed due to random acts of violence and mass hysteria.

The film also serves as a tribute to the work of seminal horror writer H. P. Lovecraft, with many references to his stories and themes. The title is a play on one of Lovecraft's most famous tales, "At the Mountains of Madness" and insanity plays a large role in the film as it does in Lovecraft's fiction. (The film's opening scene shows Trent being hauled into an asylum with the bulk of the story told in flashback, another common characteristic in Lovecraft's stories.) Also, there is a quick reference to the Old Ones of Lovecraft's Cthulhu Mythos and some locations and people (such as Mrs. Pickman) in the script are quotes as well. There is even use of direct quotes from his work in places when John Trent reads from Sutter Cane's works.

A large theme of the film is the question of free will. The main character John Trent boasts in the opening scenes of his own status as a free-lance insurance investigator: "I am my own man, no one pulls my strings but me, I'm happy." This later proves to be an ironic statement in light of his questionable existence as only a character written by Sutter Cane.

After a long dry spell in which it looked as if his career was over, Carpenter enjoyed his first spate in years of mostly positive reviews for Madness. Its box office performance was only mediocre, grossing around $10 million total.