The Eleventh Doctor is an incarnation (otherwise known as regeneration) of the Doctor, the protagonist of the British science fiction television series Doctor Who. He is portrayed by Matt Smith in three series as well as five specials. As with previous incarnations of the Doctor, the character has also appeared in other Doctor Who spin-offs both during and after the character's televised appearances.

Within the series' narrative, the Doctor is a centuries-old alien "Time Lord" from the planet Gallifrey who travels in time and space in the TARDIS, frequently with companions. At the end of life, the Doctor regenerates; as a result, the physical appearance and personality of the Doctor changes. Smith's incarnation is a quick-tempered but compassionate character whose youthful appearance is at odds with his more discerning and world-weary temperament. Preceded in regeneration by the Tenth Doctor (David Tennant), he is followed by the Twelfth Doctor (Peter Capaldi). More so than past Doctors, his episodes used time travel to tell non-sequential stories involving time travel paradoxes over a complex four year character arc.

This incarnation's main companions included Amy Pond (Karen Gillan), her husband Rory Williams (Arthur Darvill) and the mysterious Clara Oswald (Jenna Coleman). He also frequently appeared alongside River Song (Alex Kingston), a fellow time traveller with whom he shared a romantic storyline, and he was the last Doctor to appear alongside the long-serving companion Sarah Jane Smith (Elisabeth Sladen) prior to the actress's death, featuring in two episodes of the spin-off program The Sarah Jane Adventures.

The Doctor is a very special part, and it takes a very special actor to play him. You need to be old and young at the same time, a boffin and an action hero, a cheeky schoolboy and the wise old man of the universe. As soon as Matt walked through the door, and blew us away with a bold and brand new take on the Time Lord, we knew we had our man.

David Tennant announced at the National Television Awards on 29 October 2008 that he would be quitting the show, in 2010, because he felt that the four years he spent portraying the Doctor was enough. At the time, BBC News published that Paterson Joseph, who appeared in the Doctor Who episodes "Bad Wolf" and "The Parting of the Ways", was the bookmakers' favourite to succeed Tennant, followed by David Morrissey, who would be appearing in the 2008 Christmas special, "The Next Doctor". Other candidates included Sean Pertwee, son of Third Doctor actor Jon Pertwee; Russell Tovey, who portrayed Alonso Frame in the 2007 Christmas special, "Voyage of the Damned"; and James McAvoy.

Although Steven Moffat anticipated choosing a middle-aged actor for the new Doctor, Smith was aged 26 when cast, making him the youngest actor to portray the Doctor, three years younger than Peter Davison was at the time he began his role as the Fifth Doctor. Show producers were cautious about casting Smith because they felt that a 26-year-old could not play the Doctor adequately; BBC Wales Head of Drama Piers Wenger shared the sentiment, but noted that Smith was capable enough to play the role. Smith's casting in the role was revealed during an episode of Doctor Who's companion show Doctor Who Confidential, during which he described the role as "a wonderful privilege and challenge that I hope I will thrive on".

Prior to auditioning for the role, Smith had never seen Doctor Who. He prepared for the role by writing his own stories featuring the Doctor, and took special inspiration from Patrick Troughton's portrayal of the Second Doctor.

The Eleventh Doctor spends most of his first full episode, "The Eleventh Hour", in the tattered remains of the Tenth Doctor's clothing, leading young Amelia Pond to nickname him "the Raggedy Doctor."