UFO is a 1970 British science fiction television series about the covert efforts of an international defence organisation (under the auspices of the United Nations) to prevent an alien invasion of Earth. It was created by Gerry Anderson and Sylvia Anderson with Reg Hill, and produced by the Andersons and Lew Grade's Century 21 for Grade's ITC Entertainment company.

A single series of 26 episodes (including the pilot) was filmed over the course of more than a year; a five-month production break was caused by the closure of MGM-British Studios in Borehamwood, where the show was initially made. Production then moved to Pinewood Studios in Buckinghamshire. UFO was first broadcast in the UK and Canada from 1970, and in the United States from 1972.

The Andersons' live-action science fiction movie Doppelgänger (also known as Journey to the Far Side of the Sun) is considered an immediate precursor to UFO, which was their first entirely live-action TV series. (Their previous shows had used marionettes.) The series featured actors, costumes, props, locations and music that had appeared in the film, and 11 cast members of the film appeared in at least one episode of UFO.

Following syndication in the US and initial favourable ratings, a possible second series was planned; initially entitled UFO 1999, this eventually became Space: 1999, but with a totally different cast from UFO.