X the Unknown is a 1956 British science fiction horror film directed by Leslie Norman and starring Dean Jagger and Edward Chapman. It was made by the Hammer Film Productions company and written by Jimmy Sangster. The film is significant in that "it firmly established Hammer's transition from B-movie thrillers to out-and-out horror/science fiction" and, with The Quatermass Xperiment (1955) and Quatermass 2 (1957), completes "an important trilogy containing relevant allegorical threads revealing Cold War anxieties and a diminishing national identity resulting from Britain's decrease in status as a world power".

Dean Jagger was an American film, stage, and television actor who won the Academy Award for Best Supporting Actor for his role in Henry King's Twelve O'Clock High (1949).