Recurring
Arye Gross as Dr. Sidney Perlmutter, a medical examiner who assists on some of Beckett's cases and does little to hide his dislike of Castle
Scott Paulin as Jim Beckett, Kate Beckett's father
Maya Stojan as Tory Ellis, an NYPD Tech officer who assists on Beckett's cases
Juliana Dever as Jenny Ryan (née O'Malley), Ryan's wife (and Seamus Dever's real wife)
Darby Stanchfield as Meredith Harper, an actress who was Castle's first wife and Alexis's mother. She is regularly portrayed as self-centered and dramatic.
Monet Mazur as Gina Cowell, Castle's publisher and second wife
Bailey Chase as Will Sorenson, an FBI agent and Beckett's ex-boyfriend
Michael Trucco as Detective Tom Demming, a robbery detective with whom Beckett briefly becomes involved
Victor Webster as Dr. Josh Davidson, Beckett's motorcycle-riding, cardiac-surgeon ex-boyfriend
Michael Mosley as Jerry Tyson/the Triple Killer (3XK), a methodical and meticulous serial killer who returns to New York every few years to terrorize the city. He is frequently believed to be dead, only to resurface.
Jack Coleman as William Bracken, a popular, charismatic, and murderously corrupt senator representing the State of New York with ambitions of one day becoming president
Geoff Pierson as Mr. Smith, a friend of Captain Montgomery's who holds evidence that keeps the murderer of Beckett's mother away from her
Michael Dorn as Dr. Carver Burke, a psychiatrist who helps Beckett overcome the trauma of her near-fatal shooting and also deal with her various hidden emotions
Jonathan Adams as Vulcan Simmons, a ruthless Manhattan drug lord
Dan Castellaneta as Judge Markway, a judge who frequently issues warrants for Beckett, and one of Castle's poker buddies
Phil LaMarr as Dr. Holloway, a psychiatrist who evaluates the mental stability of suspects
Ken Baumann as Ashley, Alexis' steady boyfriend
Myko Olivier as Pi, Alexis's boyfriend (at the beginning of season six) with whom she shares an apartment for a while
Lisa Edelstein as Agent Rachel McCord, Beckett's partner during her brief time working for the Attorney-General's Office
Annie Wersching as Dr. Kelly Nieman, a respected plastic surgeon with a secret life as Jerry Tyson's partner in crime
Matt Letscher as 'Henry Jenkins' (real name unknown), an impostor purportedly with the CIA who is involved with Castle's disappearance
Sunkrish Bala as Vikram Singh (season 8), a high-strung tech analyst with the Attorney General's Office in Washington, D.C., where Beckett briefly works. He helps Beckett track down and destroy the mysterious organization behind Beckett's mother's murder and numerous other crimes. He later works as a computer analyst for the NYPD.
Kristoffer Polaha as Caleb Brown, an idealistic public defender found to have links to the LokSat conspiracy
James Brolin as Jackson Hunt, an operative of the Special Activities Center of the CIA, and Castle's father (from a one-night stand)
Real-life writers Stephen J. Cannell, James Patterson, Dennis Lehane, and Michael Connelly appear as themselves during periodic games of poker at Castle's apartment. Typically, they discuss Castle and Beckett's current case and tease Castle about his involvement with Beckett. Following Cannell's death on September 30, 2010, an empty chair was kept at the poker table for a year in his honor.