X the Unknown is a British science-fiction / horror film made by the famous Hammer Films company and released in 1956. The film was originally intended by Hammer to be a sequel to the previous year's successful The Quatermass Xperiment, but writer Nigel Kneale refused permission for the character of Quatermass to be used.

The film starred Dean Jagger, Edward Chapman and Leo McKern. It was written by Jimmy Sangster, and was originally to have been directed by Joseph Losey (working under the name Joseph Walton). Losey was an American director who had moved to the UK after being placed on the Hollywood blacklist. Although Losey did begin shooting the film and some of his footage is included in the final cut, when star Dean Jagger arrived on set he refused to work with a suspected Communist sympathiser. Losey was replaced by Leslie Norman, officially for health reasons.

The plot of the film sees a remote British Army base in Scotland terrorised by an unknown radioactive entity that lives underground. As the creature continues to grow in size by feeding off radioactive energy, a team of scientists are called in to investigate and stop it before it becomes too late.