The Quatermass Xperiment (a.k.a. The Creeping Unknown in the United States) is a 1955 British science fiction horror film from Hammer Film Productions, based on the 1953 BBC Television serial The Quatermass Experiment written by Nigel Kneale. The film was produced by Anthony Hinds, directed by Val Guest, and stars Brian Donlevy as the titular Professor Bernard Quatermass and Richard Wordsworth as the tormented Carroon. Jack Warner, David King-Wood, and Margia Dean appear in co-starring roles.

The film concerns three astronauts who have been launched into space aboard a single-stage-to-orbit rocket designed by Professor Quatermass. It crashlands with only one of its original crew, Victor Carroon (Richard Wordsworth), still aboard. He begins mutating into an alien organism, which, if it spawns, will engulf the Earth and destroy humanity. After Carroon escapes from custody Quatermass and Inspector Lomax (Jack Warner) of Scotland Yard have just hours to track him/it down and prevent a catastrophe.

Like its source TV serial, the film was a major success in the UK. It also brought public attention to Hammer Film Productions around the world. The film was released in the United States in a double feature with The Black Sleep.