Gallifrey is a fictional planet in the long-running British science fiction television series Doctor Who. The planet is home to both the Doctor, and others of his race, the Time Lords. It is supposed to be located in the constellation of Kasterborous, at "galactic coordinates ten-zero-eleven-zero-zero by zero-two from galactic zero centre", some 250 million light years away from Earth.

Time Lord society was founded at an unknown time in the distant past by Rassilon, whose legacy is seen throughout the entire culture and technology of the Time Lords. Perhaps the most significant of these artifacts is the Eye of Harmony, an artificial black hole created by the Time Lord Omega and captured by Rassilon for use as a power source for time travel. All Time Lord TARDIS time machines draw their power from the Eye of Harmony.

It is never definitively stated "when" the events of the Doctor's appearances of Gallifrey are set. As the planet is often reached by means of time travel it could conceivably exist anywhere in the past or future relative to our present. The planet is also surrounded by an impenetrable force field called the transduction barrier. This prevents all outsiders (with hostile intent, or otherwise) from approaching the planet and allows the Time Lords to maintain their status of absolute neutrality, letting them observe the actions of the rest of the Universe without actually taking part in its affairs. The barrier was breached once during the course of the series by the Sontarans, using an inside agent.

Gallifrey is the name of the planet as well as its major city, which houses the citadel of the Time Lords, known as the Panopticon, under which the Eye of Harmony is kept. Also on Gallifrey is the Matrix, the vast extradimensional computer network which acts as the repository of all Time Lord knowledge as well as containing the memories of dead Time Lords.

Various spin-off novels and audio plays have expanded on the history and nature of Gallifrey, although many fans do not consider the information in them to be canon. In the BBC Books novel The Ancestor Cell, Gallifrey is destroyed as a result of the Eighth Doctor's desire to prevent the voodoo cult Faction Paradox from starting a war against the unnamed Enemy. This also retroactively wiped the Time Lords from history.

It is unclear what the attitude of the new Doctor Who television series will be toward the information in the novels and audio plays, the latter produced by Big Finish Productions. However, a number of writers of the novels and audio plays are also writing for the new television series.

Gallifrey is also the umbrella title of a series of audio plays set in the Doctor Who universe, also produced by Big Finish, featuring Louise Jameson as Leela and Lalla Ward as Romana.