The former Lost star will play an FBI agent in the 19th episode of the sci-fi drama's fourth season, according to TVLine.
Cusick is best known for playing Desmond Hume on ABC's Lost from 2005 to 2010 and has since played a recurring role on Law & Order: SVU.
He will also appear opposite Kerry Washington in Shonda Rhimes's new ABC drama Scandal, which premieres on April 5.
Michael Massee also recently signed up to appear in the fourth season of Fringe. The Se7en actor will play a "memorable guest bad guy" in the 15th episode, expected to air in March.
The series continues on Fridays at 9/8c on Fox. In the UK, the fourth season will resume on Wednesday, February 8 at 10pm on Sky1.
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Watch a clip from the most recent Fringe below: