Torchwood is a British-American science fiction television programme created by Russell T Davies. A spin-off of the 2005 revival of Doctor Who, it aired from 2006 to 2011. The show shifted its broadcast channel each series to reflect its growing audience, moving from BBC Three to BBC Two to BBC One, and acquiring American financing in its fourth series when it became a co-production of BBC One and Starz. Torchwood is aimed at adults and older teenagers, in contrast to Doctor Who's target audience of both adults and children. As well as science fiction, the show explores a number of themes, including existentialism, LGBTQ+ sexuality, and human corruptibility.

Torchwood follows the exploits of a small team of alien-hunters who make up the Cardiff-based, fictional Torchwood Institute, which deals mainly with investigating incidents involving extraterrestrials. Its central character is Captain Jack Harkness (John Barrowman), an immortal con-man from the distant future; Jack originally appeared in the 2005 series of Doctor Who. The initial main cast of the series consisted of Gareth David-Lloyd, Burn Gorman, Naoko Mori, and Eve Myles. Their characters are specialists for the Torchwood team, often tracking down aliens and defending the planet from alien and human threats. In its first two series, the show uses a time rift in Cardiff as its primary plot generator, accounting for the unusual preponderance of alien beings in Cardiff. In the third and fourth series, Torchwood operate as fugitives. Gorman and Mori's characters were written out of the story at the end of the second series. Recurring actor Kai Owen was promoted to the main cast in series three, in which David-Lloyd was written out. Subsequently, American actors Mekhi Phifer, Alexa Havins and Bill Pullman joined the cast of the show for its fourth series.

The first series premiered on BBC Three and on BBC HD in 2006 to mixed reviews but viewing figures broke records for the digital channel. It returned in 2008 where it aired first on BBC Two, receiving a higher budget; its uneven tone, a criticism of the first series, was largely smoothed out, and the show attracted higher ratings and better reviews. The third series' episodes worked with a higher budget, and it was transferred to the network's flagship channel, BBC One, as a five-episode serial titled Torchwood: Children of Earth. Although Children of Earth was broadcast over a period of five consecutive summer weeknights, the series received high ratings in the United Kingdom and overseas. A fourth series, co-produced by BBC Wales, BBC Worldwide and American premium entertainment network Starz aired in 2011 under the title Torchwood: Miracle Day. Set both in Wales and the United States, Miracle Day fared less well with critics than Children of Earth, although it was applauded by some for its ambition. The series entered an indefinite hiatus after Miracle Day due to Davies' personal circumstances.

All four televised series have been broadcast in Asia, Australia, New Zealand, Europe and North America. Owing to the early popularity of Torchwood, various tie-in media were produced, including audio dramas, novels and comic strips. From its inception, the BBC invested in a heavy online presence for the series, with an alternate reality game running alongside the show's first two series, and an animated Web series running alongside its fourth. The BBC continued to approve and commission licensed spin-offs after the show's conclusion, including an audio series continuation from Big Finish Productions (2015–present).

On 21 February 2020, all 41 televised episodes returned to the BBC's online streaming service, BBC iPlayer. In the United States, the entire series was made available on HBO's new streaming service, HBO Max, upon its launch in May 2020.