The actor, best known for playing Noah Bennet in the superhero series, will be the main antagonist of Castle (Nathan Fillion) and Beckett (Stana Katic) when the show returns.
Coleman's character is a "well-liked US senator who finds himself in conflict with Beckett", Castle creator Andrew Marlowe told TVLine.
Coleman previously played State Senator Rob Lipton in The Office. He has also appeared as Bill Forbes in The Vampire Diaries and Steve Carrington in Dynasty.
Fillion recently told Digital Spy that he hopes Castle will "end strong" and not "die a slow death".
Castle was renewed for a fifth season by ABC back in May.
Watch Digital Spy chat to Nathan Fillion below: