Martha Jones is a fictional character played by Freema Agyeman in the long-running British science fiction television series Doctor Who and its spin-off series, Torchwood. The show's first female black companion, she is a companion of the Tenth Doctor in Doctor Who, after Rose Tyler (Billie Piper) but before Donna Noble (Catherine Tate). According to the character's creator Russell T Davies in his non-fiction book Doctor Who: The Writer's Tale, Martha was developed from the beginning with the intention of appearing for the whole of the 2007 series, and to make guest appearances in subsequent series and crossover appearances in the show's two spin-offs; Martha subsequently made guest appearances in Torchwood series two and in Doctor Who series four in 2008 and special episode "The End of Time" in 2010. Martha was also intended to make guest appearances in the 2009 series of Torchwood and The Sarah Jane Adventures, but could not due to the actress's other work commitments.

Within the series' narrative, Martha begins as a medical student who becomes the Doctor's time travelling companion after an incident at the hospital where she works. After more than a year of travelling with the Doctor (from both their perspectives), Martha parts from the Doctor's company as she recognises how unhealthy their relationship has become. After returning to life on Earth, becoming engaged and finishing her medical degree, Martha finds a newfound level of independence when she is recruited into the paranormal military organisations UNIT and, briefly, Torchwood. Having faced the end of the world single-handedly during her time with the Doctor, Martha is recognised for her skills both in the field and in medicine.

Freema Agyeman's first appearance in Doctor Who was in the second series (2006) episode, "Army of Ghosts", where she played Adeola Oshodi, Martha's cousin.

Martha Jones is introduced in the third series (2007) of Doctor Who, first appearing in the episode "Smith and Jones". When the hospital she works at is teleported to the Moon, medical student Martha helps save the day alongside an alien time traveller known only as the Doctor (David Tennant). To thank her for her help, the Doctor invites her to join him for a supposed single trip in his time machine the TARDIS, but later accepts her as his full-time "companion", admitting that she was "never just a passenger", and he even gives her the key to the TARDIS in the episode, "42". Martha becomes frustrated because the Doctor is oblivious to her feelings for him, and she expresses concern that she is simply a rebound after the Doctor's painful loss of his previous companion, Rose Tyler (Billie Piper). When the amnesiac Doctor falls in love in the two-part story "Human Nature" and "The Family of Blood", a pained Martha laments "You had to go and fall in love with a human... and it wasn't me". In the series finale, "The Sound of Drums" and "Last of the Time Lords", in which the Doctor's nemesis the Master (John Simm) takes over planet Earth, capturing both the Doctor and fellow companion Captain Jack Harkness (John Barrowman), Martha manages to escape by teleporting away but is left alone to save the world. On the run from the Master, she spends a year travelling the world in a plan which restores the incapacitated Doctor and reverses time, undoing the Master's actions. Able to remember the events during the Master's reign, Martha then leaves the TARDIS of her own accord, telling the Doctor that she cannot waste her life pining for someone when the relationship cannot happen, but promises that she will see him again. Martha, as voiced by Freema Agyeman, also appears in the 2007 animated serial The Infinite Quest, which aired in twelve weekly segments during the run of the 2007 series.

Martha reappears in the second series (2008) of the Doctor Who spin-off, Torchwood, which focuses on Captain Jack Harkness. First appearing in the episode "Reset" as part of a three-episode story arc, Martha has been temporarily drafted to the Torchwood organisation of alien-hunters by Jack, requiring a medical expert on alien life. Through exposition, it is revealed that Martha has become a "medical officer" for the international paranormal investigations agency UNIT since qualifying as a Doctor of Medicine. Martha briefly joins the Cardiff-based Torchwood Three as its medical officer following the death of Owen Harper (Burn Gorman) but later leaves the organisation in the episode "A Day in the Death" once she is satisfied that Owen is fit to return to duty following his resurrection. Later in the fourth series of Doctor Who (2008), Martha returns for a three-episode arc beginning with the two-part story, "The Sontaran Stratagem" and "The Poison Sky", and ending with "The Doctor's Daughter", in which she meets the Doctor's new companion Donna Noble (Catherine Tate); in the first episode, a more assertive and engaged Martha summons the Doctor to Earth to help uncover a plot by the Sontarans. Martha returns again for the final two episodes of the series, "The Stolen Earth" and "Journey's End", where she has been promoted to a U.S. division of UNIT and is working on a top-secret teleportation project based on Sontaran technology. She rallies alongside fellow companions Jack and Sarah Jane Smith (Elisabeth Sladen) in an effort to face the threat of Davros' (Julian Bleach) plot to destroy reality. In facing Davros, Martha threatens to set off nuclear warheads which will destroy the Earth in order to spare human suffering and curtail his plans, but is stopped by the Doctor. In the episode's dénouement, Martha leaves with Jack and former companion Mickey Smith (Noel Clarke), with Jack saying to her, "I'm not sure about UNIT these days... maybe there's something else you could be doing."

Despite the set-up at the end of Doctor Who series four, Martha does not appear in Torchwood: Children of Earth (2009). Martha's absence is explained when the characters interact with UNIT officers in Children of Earth; she is on her honeymoon. In lieu of Martha, the character of Lois Habiba (Cush Jumbo) was created. A scene in "The End of Time" (2010) shows Martha, apparently having left UNIT, fighting aliens with Mickey and marrying him rather than her previous fiancé. The Doctor appears to the pair shortly before his pending regeneration to save them from a Sontaran sniper. Agyeman is credited as portraying Martha Smith-Jones.

Aside from television appearances, the character of Martha also appears in Doctor Who novels and comic books, some of which are ambiguous in terms of their canonicity in relationship to the television series. In books, Martha appears in the "New Series Adventures" series of Doctor Who novels, published by BBC Books. The first book published was a "Quick Reads" novel, Made of Steel by Terrance Dicks (published prior to her first television appearance), and the character subsequently appeared in all novels in the series, starting with Sting of the Zygons by Stephen Cole and most recently in The Many Hands by Dale Smith. Freema Agyeman physically represents the character on the cover of every novel. In late 2008 The Story of Martha, a collection of stories focusing on Martha's adventures between "The Sound of Drums" and "Last of the Time Lords" was published.

In terms of comic book appearances, Martha has appeared in the Doctor Who Magazine strips from #381 onwards and the Doctor Who Adventures comics from #28 onwards. The character also periodically appears in the Battles in Time series of comic books. In 2007, American comic book publisher IDW Publishing (publisher of various Angel, Star Trek and The Transformers comic titles) announced their plans to do a devoted series of Tenth Doctor and Martha comics for an American audience. When asked about canonicity, IDW executive editor Chris Ryall dodged the issue by saying all the comics are "blessed" by Russell T Davies but it is up to the individual how canonical each story is.