Dancy will play John Zabel, a savvy Washington consultant who joins the White House as a new foreign-policy advisor to the President and a formidable opponent to Saul Berenson (played by Mandy Patinkin).
The show is currently in production and will premiere later this year.
The final season will see Carrie Mathison (Danes) recovering from months of brutal confinement in a Russian gulag. Her body is healing, but her memory remains fractured.
Meanwhile, Saul has been dispatched to engage the Taliban in peace negotiations, and against medical advice, he asks Carrie to walk with him into the lion's den one last time.
The role marks Dancy's return to Showtime after he appeared in season two of "The Big C". He last starred opposite Aaron Paul in Hulu's "The Path".
"Homeland", developed by Gansa and Howard Gordon, is based on the original Israeli series "Prisoners of War" by Gideon Raff.
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