The Quatermass Experiment is a British science fiction serial broadcast by BBC Television during the summer of 1953 and re-staged by BBC Four in 2005. Set in the near future against the background of a British space programme, it tells the story of the first crewed flight into space, supervised by Professor Bernard Quatermass of the British Experimental Rocket Group.

When the spaceship that carries the first successful crew returns to Earth, two of the three astronauts are missing, and the third – Victor Carroon – is behaving strangely. It eventually becomes apparent that an alien presence entered the rocket during its flight, and Quatermass and his associates must prevent the alien from destroying the world.

Originally comprising six half-hour episodes, it was the first science fiction production to be written especially for a British adult television audience. The serial was the first of four Quatermass productions to be screened on British television between 1953 and 1979. It was transmitted live from the BBC's original television studios at Alexandra Palace in north London, one of the final productions before BBC television drama moved to west London.

Despite its success and influence, only two episodes have survived, the other four having never even been recorded on their live broadcast. As well as spawning various remakes and sequels, The Quatermass Experiment inspired much of the television science fiction that succeeded it, particularly in the United Kingdom, where it influenced successful series such as Doctor Who and Sapphire and Steel. It also influenced successful Hollywood films such as 2001: A Space Odyssey and Alien.