Which Doctor regenerated in 'The Parting of the Ways'?

The Ninth Doctor regenerated in “The Parting of the Ways.” Christopher Eccleston made his final appearance as the Ninth Doctor in this 2005 Doctor Who episode, which ends with his transformation into the Tenth Doctor, played by David Tennant.

The regeneration follows the Doctor’s exposure to the immense energy absorbed by Rose Tyler from the heart of the TARDIS. After Rose destroys the Dalek fleet and resurrects Captain Jack Harkness, the Doctor takes the dangerous energy from her body. This saves Rose but leaves the Doctor dying.

“The Parting of the Ways” was the thirteenth and final episode of Doctor Who’s revived first series. It was first broadcast on 18 June 2005 and concludes the two-part story that begins with “Bad Wolf.” The episode also marks the first appearance of David Tennant as the Tenth Doctor.

A common mix-up is confusing this event with the Tenth Doctor’s later regeneration in “The End of Time,” when he becomes the Eleventh Doctor. The regeneration in this episode is specifically from the Ninth Doctor to the Tenth.