"The Waters of Mars" is the second of four hour-long specials of the British science fiction television series Doctor Who, all serving as David Tennant's final episodes as the Tenth Doctor. As with the previous special, "Planet of the Dead", it was simultaneously broadcast on BBC One and BBC HD on 15 November 2009.

Set on Mars in the year 2059, the episode depicts the Doctor encountering the first human colony, Bowie Base One, commanded by one-off companion Captain Adelaide Brooke (Lindsay Duncan), who turns out to be a pivotal character in the history of humanity. The Doctor must decide whether to use his knowledge of her fate to change history.

Originally written as a Christmas special entitled "Red Christmas", starring Helen Mirren as Adelaide, the episode according to head writer and executive producer Russell T Davies, the special is closely linked to the next two episodes but is not the first part of a three-part story.

The special was dedicated to Barry Letts, the former writer and producer of Doctor Who who died in October 2009. The episode won the 2010 Hugo Award for Best Dramatic Presentation, Short Form.

"The Waters of Mars" was first broadcast on a Sunday, the only non-Christmas episode of the revived series to air outside the usual Saturday evening slot, until "The Woman Who Fell to Earth", in 2018. Since its first broadcast, the special has received critical acclaim from critics and audiences for the dark tone, Tennant's performance, the writing, and the character development of the Tenth Doctor, with many considering it to be one of the show's darkest and best episodes. The special was watched by 10.32 million viewers, becoming the fifth most watched programme of the week as well as the third most watched programme of the day, behind ITV1's The X Factor and I'm a Celebrity...Get Me Out of Here!.