The Red House Mystery is a "locked room" whodunnit by A. A. Milne, published in 1922. It was Milne's only mystery novel.

The story is set in the quaint, English countryside at the house party of Mark Ablett, where a murder quickly takes place. Ablett has been entertaining a house party consisting of a widow and her marriageable daughter, a retired major, a willful actress, and Bill Beverley, a young man about town. Mark's long-lost brother Robert, the black sheep of the family, arrives from Australia and shortly thereafter is found dead, shot through the head.

Mark Ablett has disappeared, so Tony Gillingham, a stranger who has just arrived to call on his friend Bill, decides to investigate. Gillingham plays Sherlock Holmes to his younger counterpart's Doctor Watson; they progress almost playfully through the novel while the clues mount up and the theories abound.

In his introduction to the 1926 UK edition, A. A. Milne said he had "a passion" for detective stories, having "all sorts of curious preferences" about them: though in real life the best detectives and criminals are professionals, Milne demanded that the detective be an unscientific amateur, accompanied by a likeable Watson, rubbing shoulders with an amateur villain against whom dossiers and fingerprints are of no avail.

This was Milne's first and final venture into the detective and mystery genre, despite its immediate success and an offer of two thousand pounds for his next mystery novel.

Milne lets his readers inside the head of his amateur detective, disregarding the clichéd romance or violence of other detective novels, as the mystery becomes a puzzling sort of parlor game for the novel's characters and readers alike.

Alexander Woollcott called The Red House Mystery "one of the three best mystery stories of all time."