- Judge Markway (Dan Castellaneta) - a golfing friend of Castle's who periodically joins him for poker games at his apartment (appears in season 1 episodes Flowers For Your Grave and Ghosts.
- Gina Cowell (Monet Mazur) - Castle's publisher and ex-wife (appears in the series premiere Flowers For Your Grave and the season 2 finale A Deadly Game).
- Will Sorenson (Bailey Chase) - an attractive FBI Agent who dated Beckett for 6 months before she met Castle. The two broke up because of a career opportunity he was offered in Boston and Beckett remains somewhat bitter about his decision to leave New York (appears in season 1 episodes Little Girl Lost and A Death in the Family)
- Jim Beckett (Scott Paulin) - Kate Beckett's father (appears in a photo in A Chill Goes Through Her Veins from season 1 and in Sucker Punch in season 2).
- Mayor of New York City (Joseph C. Phillips) - he is a friend and fan of Castle's books who gives his consent to Castle's request to shadow Beckett at the beginning of the series. Castle has nicknamed him "Big Cheese" and, along with Judge Markway, he occasionally joins Castle for poker at his apartment (appears in season 1 episodes Home is Where the Heart Stops and Ghosts).
- Dr. Clark Murray (Robert Picardo) - a forensic pathologist known as "Dr. Death" and friend of Castle's who Castle enlists, without Beckett's knowledge or approval, to help him look into Beckett's mother's murder case (appears in the season 1 finale A Death in the Family and Sucker Punch in season 2).
- Detective Roselyn Karpowski (Diana-Maria Riva) - a colleague of Beckett's at the precinct (appears in The Double Down and When The Bough Breaks, both season 1).
- Gilbert Mazzarra (Danny Nucci) - a detective friend and former colleague of Esposito's (appears in Love Me Dead and Sucker Punch, both season 2).
- Dr. Holloway (Phil LaMarr) - a psychologist with the NYPD with whom Castle and Beckett consult on a couple cases (appears in Vampire Weekend and The Fifth Bullet, both season 2).