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K-9, or K9, is the name of several fictional robot canine (the name being a pun on that word) in the long-running British science fiction television series, Doctor Who, first appearing in 1977. K-9 has also been in a central character in three of the series television spin-offs, the unsuccessful K-9 and Company (1981), The Sarah Jane Adventures (2007–present) and K-9 (2009–present). Although not originally intended to be a recurring character in the series, K-9 was kept in the show following his first appearance because he was expected to be popular with younger audiences. There have been at least four separate K-9 units in the series, with the first two being companions of the Fourth Doctor. Voice actor John Leeson has provided the character's voice in most of his appearances, except during Season 17 of Doctor Who, in which David Brierley temporarily provided the character's voice. The character was created by Bob Baker, to whom rights to the character still belong; consequently, Baker's spin-off series K-9, which is not BBC-produced, cannot directly reference events or characters from Doctor Who, though it attempts to be a part of that continuity.

Within the series' narrative, K-9 is a robot dog acquired by Doctor Who ' s title character in the 1977 serial The Invisible Enemy. The first two incarnations of the character travelled alongside the Doctor, then in his fourth incarnation (portrayed by Tom Baker) until 1981. In these stories, K-9 proved useful for the powerful laser weapon concealed in his nose, his encyclopaedic knowledge and his vast computer intelligence. By 1981, each of the two models of K-9 which travelled alongside the Doctor had been left with one of the Doctor's female companions. The character subsequently transitioned into spin-off territory. Producers hoped K-9's popularity with children would launch the series K-9 and Company, led by actress Elisabeth Sladen in the role of companion Sarah Jane Smith alongside a new K-9. In 2006, K-9 reappeared in the revived series of Doctor Who. Although the character's appearances in the 2007 spin-off The Sarah Jane Adventures were hampered by the concurrent development of the K-9 spin-off, the character began to appear full-time in both K-9 and The Sarah Jane Adventures, depicting adventures of the original K-9 Mark I and of Mark IV, respectively.

K-9, retroactively "K-9 Mark I" (Leeson), initially appeared in The Invisible Enemy (1977) as the creation of Professor Marius (Frederick Jaeger) in the year 5000. K-9 subsequently traveled with the Fourth Doctor (Tom Baker) and Leela (Louise Jameson) as a companion of the Doctor in his adventures in time and space until The Invasion of Time (1978). In this serial, K-9 decides to remain on the Doctor's home planet of Gallifrey with Leela. Immediately afterwards, Doctor Who would introduce a second incarnation of K-9, played by the same prop; the last scene of The Invasion of Time shows the Doctor unpacking a box labeled "K-9 Mk II". Although the first incarnation of K-9 does not appear again in televised Doctor Who media, he is the star of the 2009 K-9 television series, in which he undergoes a sort of "regeneration" process from which a new, more sophisticated and futuristic K-9 emerges; in the first series of K-9, the character is transported to London c. 2050 by Professor Gryffen (Robert Moloney). Though on regeneration the character loses his memory of his adventures with the Doctor, he assists Gryffen and several teenage companions against a dystopian regime of "The Department". The upgraded K-9 has new specifications, sporting a sophisticated head-up display, the ability to fly, and more powerful laser weapons.

K-9 Mark II first appears in The Ribos Operation (1978). More mobile than his predecessor, Mark II exhibited the ability to sense and warn others of danger. He travels as a companion alongside the Doctor and Romana (Mary Tamm, later Lalla Ward). Around the time of Romana's regeneration (the transition from actress Mary Tamm to Lalla Ward), the character was explained as suffering from "laryngitis" to accommodate Leeson's departure from the series at the start of the 1979-80 season; for this time, he was portrayed by David Brierley, until Leeson's return for the 1980-81 season. When the Doctor and Romana travel to the parallel universe of E-Space, K-9 is severely damaged, in Warriors' Gate (1981). The damage was such that K-9 could only function in E-Space; when Romana decided to stay and forge her own path, the Doctor gave K-9 to her. The character makes subsequent appearances in remakes of the unfinished serial Shada, alongside the Eighth Doctor (Paul McGann); the 2003 Shada audio play and webcast depict K-9 Mark II as having returned to Gallifrey with Romana, now Lady President of the Time Lords, where the Doctor visits them.

K-9 has not been a contender in any CBUB matches.