Doctor Who returned to British screens in 2005, with Christopher Eccleston playing the role of the Ninth Doctor in the first series. Tennant replaced him as of the second series, making his first, brief appearance as the Tenth Doctor in the episode "The Parting of the Ways" (2005) at the end of the regeneration scene, and also appeared in a special 7-minute mini-episode shown as part of the 2005 Children in Need appeal, broadcast on 18 November 2005. He began filming the new series of Doctor Who in late July 2005. His first full-length outing as the Doctor was a 60-minute special, "The Christmas Invasion", first broadcast on Christmas Day 2005. Tennant had been formally offered the role of the Doctor during rehearsals for The Quatermass Experiment. Although the casting was not officially announced until later in April, both castmates and crew became aware of the speculation surrounding Tennant; in the live broadcast Jason Flemyng (Quatermass) changed his first line to Tennant's Dr Briscoe from "Good to have you back, Gordon" to "Good to have you back, Doctor" as a deliberate reference.
Tennant has expressed enthusiasm about fulfilling his childhood dream. He remarked in a radio interview: "Who wouldn't want to be the Doctor? I've even got my own TARDIS!" In 2006, readers of Doctor Who Magazine voted Tennant "Best Doctor" over perennial favourite Tom Baker. Writer Russell T Davies made the decision not to use Tennant's own Scottish accent for the character as he did not want the Doctor's accent "touring the regions", using Estuary English instead. Tennant has gone on record as saying that, contrary to tabloids reports, he was not upset at not being able to play the role in his own accent and in fact had never wanted to. However he was pleased to be able to use his own accent in one episode, when the Doctor briefly masquerades as "Dr James McCrimmon" of Edinburgh in Tooth and Claw – a nod to the Second Doctor's companion Jamie McCrimmon.
He previously had a small role in the BBC's animated Doctor Who webcast Scream of the Shalka. Not originally cast in the production, Tennant was recording a radio play in a neighbouring studio, and when he discovered what was being recorded next door convinced the director to give him a small role. This personal enthusiasm for the series had also been expressed by his participation in several audio plays based on the Doctor Who television series which had been produced by Big Finish Productions, although he did not play the Doctor in any of these productions. His first such role was in the Seventh Doctor audio Colditz, where he played a Nazi lieutenant guard at Colditz Castle. In 2004 Tennant played a lead role in the Big Finish audio play series Dalek Empire III as Galanar, a young man who is given an assignment to discover the secrets of the Daleks. In 2005, he starred in UNIT: The Wasting for Big Finish, recreating his role of Brimmicombe-Wood from a Doctor Who Unbound play, Sympathy for the Devil. In both audio productions, he worked alongside Nicholas Courtney, who reprised the character of Sir Alastair Gordon Lethbridge-Stewart. He also played an unnamed Time Lord in another Doctor Who Unbound play Exile. UNIT: The Wasting, was recorded between Tennant getting the role of the Doctor and it being announced. He played the title role in Big Finish's adaptation of Bryan Talbot's The Adventures of Luther Arkwright (2005). In 2006, he recorded abridged audio books of The Stone Rose by Jacqueline Rayner, The Feast of the Drowned by Stephen Cole and The Resurrection Casket by Justin Richards, for BBC Worldwide.
He made his directorial debut on the Doctor Who Confidential episode that accompanies Steven Moffat's episode "Blink", entitled "Do You Remember The First Time?", which aired on 9 June 2007. In 2007, Tennant's Tenth Doctor appeared with Peter Davison's Fifth Doctor in a Doctor Who special for Children in Need, written by Steven Moffat and entitled "Time Crash". He later performed alongside Davison's daughter, Georgia Moffett (as "Jenny") in the 2008 episode "The Doctor's Daughter".
Tennant featured as the Doctor in an animated version of Doctor Who for Totally Doctor Who, The Infinite Quest, which aired on CBBC. He also starred as the Doctor in another animated six-part Doctor Who series, Dreamland. Tennant guest starred as the Doctor in a two-part story in Doctor Who spin-off The Sarah Jane Adventures, broadcast in October 2009. He continued to play the Tenth Doctor into the revived programme's fourth series in 2008. However, on 29 October 2008, he announced that he would be stepping down from the role after three full series. He played the Doctor in four special episodes in 2009, before his final episode aired on 1 January 2010, where he was replaced by the eleventh doctor, portrayed by Matt Smith.
Tennant and Billie Piper returned to Doctor Who for the 50th anniversary special, "The Day of the Doctor" broadcast on 23 November 2013, with then-stars Matt Smith and Jenna-Louise Coleman and guest star John Hurt. The same month, he also appeared in the one-off 50th anniversary comedy homage The Five(ish) Doctors Reboot directed by Peter Davison.
In October 2015, Big Finish Productions announced that Tennant would return to the role of the Tenth Doctor alongside Catherine Tate as his former companion Donna Noble in three new stories from Big Finish. The stories feature current and previous Doctor Who actors, including Strax actor Dan Starkey, former Davros actor Terry Molloy, and many veterans of Big Finish, including Niky Wardley, who portrayed Eighth Doctor companion Tamsin. The three stories were released in May 2016.
In November 2017, three new audio dramas were released by Big Finish Productions with Tennant once again starring as the Tenth Doctor, alongside Billie Piper as Rose Tyler.