In which episode did the Tenth Doctor regenerate?

The Tenth Doctor regenerated in The End of Time, specifically the story’s second and final episode. David Tennant’s Doctor became Matt Smith’s Eleventh Doctor at the story’s conclusion.

The End of Time was broadcast as a two-part Doctor Who story during the 2009–10 holiday period. Part One aired on 25 December 2009, while Part Two aired on 1 January 2010. The story was written by Russell T Davies and marked his final regular Doctor Who script before his later return.

The regeneration follows the Doctor’s decision to save Wilfred Mott from lethal radiation inside a Vinvocci control chamber. Although the Doctor survives the immediate crisis, the radiation poisoning is fatal. He says goodbye to several former companions before returning to the TARDIS.

Viewers sometimes confuse this with “The Parting of the Ways,” where the Ninth Doctor regenerates, or “The Time of the Doctor,” where the Eleventh Doctor regenerates. The Tenth Doctor’s final scene ends with the famous line, “I don’t want to go.”