Morshower will play Michael Reed, a former general who will stop at nothing to protect the secrets of his past. With Reed's military connection, could he be involved in Washington, where Beckett (Stana Katic) was offered a job in the Season 5 finale?
Morshower, who recurred on 24 as Secret Service agent Aaron Pierce, most recently had a multi-episode arc on NBC's Revolution.
Castle returns on Monday, Sept. 23 at 10/9c on ABC.
Additional reporting by Adam Bryant.