Denisof and Greenwalt previously worked together on the supernatural drama "Angel."
TVLine said the actor will play Viktor Albert Wilhelm George Beckendorf, the first cousin of James Frain's Renard character, on "Grimm."
The show's third season is to start Oct. 25.
Also known for his work on "Buffy the Vampire Slayer," Denisof is the husband of actress Alyson Hannigan.
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