The Doctor, sometimes known as Doctor Who, is the protagonist of the long-running BBC science fiction television series Doctor Who. An extraterrestrial Time Lord, the Doctor travels the universe in a time travelling spaceship called the TARDIS, often with companions. Since the show's inception in 1963, the character has been portrayed by fourteen lead actors. The transition to each succeeding actor is explained within the show's narrative through the plot device of regeneration, a biological function of Time Lords that allows a change of cellular structure and appearance with recovery following a mortal injury.

A number of other actors have played the character in stage and audio plays, as well as in various film and television productions. The Doctor has also been featured in films and a vast range of spin-off novels, audio dramas and comic strips.

Ncuti Gatwa most recently portrayed the Fifteenth Doctor from "The Giggle" (2023) up to "The Reality War" (2025).

The Doctor is a Time Lord who travels through time and space in a dimensionally transcendental – "bigger on the inside" – time machine: the TARDIS. This time machine, whose name is an acronym for Time And Relative Dimension(s) In Space, takes the exterior form of a 1963 police telephone call box and retains the appearance throughout the programme. Human companions accompany the Doctor through their adventures and serve as audience surrogate characters to ask questions which allow the Doctor to provide relevant exposition.

"Doctor" is a self-selected alias. In episodes specifically under showrunner Steven Moffat, the story arcs surrounding events in the Doctor's future implied serious consequences in the event of the Doctor's true name being spoken, with the nature of these finally revealed in "The Time of the Doctor". Spin-off media offer the explanation that the Doctor's true name is unpronounceable by humans. In "The Name of the Doctor", the Eleventh Doctor tells companion Clara Oswald that the name "Doctor" is essentially a promise he made. The promise itself is revealed in "The Day of the Doctor": "Never cruel nor cowardly. Never give up. Never give in."

The episode "The Timeless Children" revised the Doctor's origins, revealing a scientist and space explorer named Tecteun who found a lone, mysterious child with a supernatural physiology – one not belonging to any other life form or species – and an immense intelligence. She adopted the child and studied her, successfully grafting her regeneration capacity (and possibly other traits) into her own species, the Shobogans, and herself. This species, who would eventually become the Time Lords, was restricted to a limit of twelve regenerations by a later incarnation of Tecteun. Tecteun and their child were eventually inducted into a clandestine Time Lord organisation known as the Division. After an unknown amount of regenerations, Tecteun's child began calling themself "Doctor". The Fugitive Doctor, true to her title, was on the run from the Division in a TARDIS disguised as a police box. The details of their life were also redacted from the Matrix – only snippets remaining, masked as the story of the Irish Garda Brendan. The true origins of the Time Lords remained hidden from themselves and from the Doctor.

The First Doctor's subsequent childhood on Gallifrey has been little described in the series. In "Hell Bent" the Doctor recalled his origins as a high-born Gallifreyan. In The Time Monster, the Doctor says he grew up in a house on a mountainside and talks about a hermit who lived under a tree behind the house and inspired the Doctor when he was depressed. He is later reunited with this former mentor, now on Earth posing as the abbot K'anpo Rimpoche, in Planet of the Spiders. In "The Girl in the Fireplace", according to Madame de Pompadour who psychically linked with the Doctor's memories, the Doctor experienced a very lonely childhood. An elderly woman on Gallifrey died and was shrouded in veils and surrounded by flies, giving the Doctor recurring nightmares, which the confession dial in "Heaven Sent" would later visualise to torment him. In "Listen", it is ambiguously revealed the Doctor as a child often slept alone in a barn in the Drylands (a desert region outside the city capital), was withdrawn from other children, and was cared for by guardian figures who privately doubted the child's ability as an eventual Time Lord. Through the dialogue, it is suggested that several Gallifreyan children were pressured into joining the army, a path which did not sit right with the Doctor's pacifist beliefs, and as a result he wished to enroll into the Time Lord Academy instead.

The classic series refers to his time at the academy and his affiliation with the notoriously devious Prydonian chapter of Time Lords. In "The Sound of Drums", the Doctor describes an academy initiation where, at the age of eight, Gallifreyan children were taken from their families and made to look into the Untempered Schism, a gap in the fabric of reality, to view the Time Vortex. According to the Doctor, when regarding the effects of the initiation on participants: "Some would be inspired, some would run away and some would go mad" (as he suggests happened to his nemesis, the Master). When asked to which group he belonged, he replied, "Oh, the ones that ran away. I never stopped!" The Doctor was taught by future Lord President Borusa and Azmael, where he met Drax, with whom he attended a Tech course as part of the class of '92. In The Armageddon Factor, it is revealed that the Doctor scraped through the academy with 51% on his second attempt. In The Time Meddler, it is said that the Doctor was fifty years before the Meddling Monk. In Time and the Rani, the Doctor claims to have attended university alongside the Rani, specialising in thermodynamics.