© Rex Features
The Dollhouse star will make his first appearance in the season finale of the ABC series on May 7, playing a mysterious character named Cole Maddox.
TV Line describes Maddox as a cold, calculated individual with a military background who could potentially return for more episodes next season.
Adam Baldwin will also star in an upcoming episode of Castle as a "rough and tumble" detective.
Lead actress Stana Katic recently said that a relationship between her character Kate Beckett and Castle (Nathan Fillion) is "pretty justified".
Castle returns April 16 at 10/9c on ABC.
