- Judge Markway (Dan Castellaneta) - a golfing friend of Castle's who periodically joins him for poker games at his apartment in season 1.
- Gina Cowell (Monet Mazur) - Castle's publisher and ex-wife who appears in the series premiere and season 2 finale.
- 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. He appears in two episodes in season 1.
- Jim Beckett (Scott Paulin) - Kate Beckett's father. He appears in a photograph in one episode of season 1 and in the flesh in one episode of 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. He appears in two episodes in season 1.
- 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. He appears in the final episode of season 1 and one episode of season 2.
- Detective Roselyn Karpowski (Diana-Maria Riva) - a colleague of Beckett's at the precinct. She appears in two episodes in season 1.
- Gilbert Mazzarra (Danny Nucci) - a friend and former colleague of Esposito's within the department. He appears in two episodes in season 2.
- Dr. Holloway (Phil LaMarr) - a psychologist with the NYPD with whom Castle and Beckett consult twice. He appears in two episodes in season 2.