Flighty ex-Broadway diva and mother of Richard Castle and grandmother of Alexis Castle. She moves into Castle's loft when the man she dates steals all her money, leaving her penniless, and forcing her to resume her acting career.
Even though she still retains a love of fun, drinking, gambling, shopping, and men, she is devoted to her son and granddaughter and does what she can to use her life experiences to help them with their problems. She also connects with Kate Beckett, the homicide detective Castle shadows as inspiration for his Nikki Heat series of books. While she worries periodically about her son's safety in working with the police, she also sees that Beckett has come to mean much more to Castle than just his muse.
She is played by Susan Sullivan.
Castle repeatedly teases his mother about how she flew from one relationship to another as he was growing up, and how he lacked a stable male influence in his life. He also teases his mother about her free-wheeling, less-than-responsible lifestyle in her younger years, saying that every Sunday she made him give her an ice pack and a Bloody Mary for breakfast.
Once became paranoid that an understudy on a play she was starring in was out to poison her, so she made Castle pay for daily blood tests to be sure she would not get sick. She did eventually fall ill, after the understudy put laxative in some candy and gave it to her.
Years ago she appeared for three weeks on the fictional soap opera Temptation Lane as the love interest of one of the main characters played by actor Lance Hastings (guest star Corbin Bernsen).
Castle was in 5th grade when Martha was touring with A Chorus Line.  One summer shortly thereafter, she was on the road in Pippin.  At some point she played Maggie the Cat in Cat On A Hot Tin Roof, for which she got at least one unfavorable review.
She was once nominated for a Tony Award. 
In The Time of Our Lives, Martha has been asked to audition for a "new Stoppard play"; in the alternate timeline Castle visits, she's had a wildly successful career, which inspires him to urge her to audition once he's back in his own universe. She does get the part and in later episodes refers to wardrobe fittings and previews.
She briefly dates and moves in with Chet Palaburn, her high school sweetheart with whom she reconnects via a social networking site. Chet eventually proposes, but before Martha can turn him down, he dies suddenly of a stroke. Later, she finds out he left her one million dollars in his will, which she is reluctant to accept, but eventually does so with plans to open an acting school.
For a time, she dated her co-star Lance Hastings on Temptation Lane and rekindled their romance in One Life to Lose when Castle and Beckett investigate the murder of one of the show's writers.
Martha meets someone named Robert while attending a grief support group during Castle's absence.  She tells Castle to hide when Robert comes to pick her up for a date, as Robert thinks Castle is still missing.
One of Martha's beaux comes downstairs wearing Castle's pajamas, just as Castle is observing how crowded his loft has become. 
- "You is who you is. And if you ain't who you is, then who is you?"