The Quatermass Xperiment is a 1955 British science-fiction / horror film, based on the 1953 BBC Television serial The Quatermass Experiment by Nigel Kneale.

This feature film version was adapted (and probably largely scripted in its final form) by the director, Val Guest, working with American screenwriter Richard Landau. It was produced by the Hammer Film Productions Ltd. company, who changed the title to The Quatermass Xperiment, with the strange spelling, in order to play on the film's British X-certification (adults only) status, claiming that it was the first British-made movie to have received such a classification. In America, the film was released under the title The Creeping Unknown in 1956, distributed by United Artists as the b-film to the American-made The Black Sleep. Some prints screened on British television have used the conventional spelling of "Experiment".

The film starred American actor Brian Donlevy as Professor Bernard Quatermass, the lead role having gone to him in an attempt to appeal to the North American audience, probably at the insistence of Robert L. Lippert who helped finance the film, as he had many of Hammer's earlier films with the same casting stipulation (Lippert would distribute the British Hammers in the USA, in exchange for the Hammer distribution arm, Exclusive Films Ltd., handling Lippert's films in the UK). Other actors starring included future Dixon of Dock Green star Jack Warner as Quatermass' nemesis, Police Inspector Lomax; David King-Wood as Quatermass' space medicine specialist, and Richard Wordsworth in a memorable performance as the film's central figure, Carroon, an astronaut whose body has been invaded by an alien life-form and is being transformed into something unearthly and dangerous. Also appearing were, in a small cameo as a local drunk, Dame Thora Hird (surprisingly given fifth billing); Lippert's girlfriend Margia Dean as Carroon's wife; and Lionel Jeffries as a British bureaucrat. Jane Asher also made an uncredited appearance as a small child, her first screen role.

The film was quite popular at the time, successful enough for Hammer to produce adaptations of the following two Quatermass serials, releasing them to the cinema as Quatermass 2 (1957) and Quatermass and the Pit (1967). It is also of special interest today as a complete copy of the original BBC television version of the story no longer exists.

Quatermass creator Nigel Kneale has vociferously decried the film version of his teleplay, and is especially critical of the casting of and performance by Brian Donlevy as Bernard Quatermass. Kneale has at times alleged that Donlevy was drunk much of the time, although this claim is denied by Val Guest and others associated with the production.

The plot of the film seems to some to have been a major influence on a 1999 Johnny Depp movie The Astronaut's Wife, although it is not known whether the film really was an inspiration or whether this is merely coincidence.

Among the materials used by Les Bowie to embellish the monster were bovine entrails and tripe.

This film was originally slated to be released in the United States by 20th Century Fox. However, to convince more exhibitors to install Cinemascope equipment, studio chief, Darryl F. Zanuck, pledged that all future 20th Century Fox releases would be in Cinemascope or a compatible anamorphic process. Since this Hammer production was shot in standard academy (1.33:1), it had to be passed over. It was picked up and released through United Artists.

The original British release billed Brian Donlevy and Jack Warner before the main title (The Quatermass Xperiment), with Margia Dean heading the supporting cast after the title. For the US release, Donlevy, then Ms. Dean, and then Jack Warner were billed before the main title (The .Creeping Unknown), moving Thora Hird up to the top of the supporting cast, the remaining order of which was not changed. Also, Val Guest did not receive co-screenplay credit on the US credits, Nigel Kneale's credit was altered to simply "Based on the play by", and the Acknowledgments credits were omitted altogether.