According to TV Guide, which first broke the news on Friday (Sept. 20), the actress will play Maggie Ingram, the long-time girlfriend of a death row inmate who is on the verge of being executed. She is slated to appear in the upcoming sixth season's seventh episode, titled, "Like Father, Like Daughter."
Alexis (Molly Quinn) is said to be heavily involved in the case, along with her dad Castle (Nathan Fillion) and Beckett (Stana Katic).
Carter portrays Ava, the wife and co-conspirator of criminal kingpin Boyd Crowder (Walton Goggins) on the FX drama "Justified." The character ended Season 4 in jail for crimes that she committed with her husband.
Her other recent TV credits include "Grey's Anatomy," "Body of Proof" and "Prime Suspect."
"Castle" Season 6 is scheduled to premiere this Monday, Sept. 23 at 10/9 p.m. CT on ABC, while "Justified" returns to FX for Season 5 early next year.