The former Jericho star will play a Wildermann, a creature similar to the legendary beast Big Foot.
The character's real-world alter ego Larry struggles with self-control and switches between his human and animal forms, reports TVLine.
Canadian actor Mitchell recently appeared in episodes of The Mentalist, Castle and Private Practice. He also starred in ABC pilot Partners last year, but the cop drama was not picked up to series.
He will appear in the 21st episode of Grimm's first season, which is scheduled to air in May.
David Zayas was also recently cast on the supernatural drama. The Dexter star will play a monstrous, troll-like creature known as a Hasslich.
Grimm follows Nick Burkhardt (David Giuntoli), a homicide detective who wards off paranormal beings.
The series recently became one of NBC's first renewals, receiving a second season pick-up.
Grimm airs on Fridays at 9/8c on NBC. In the UK, the show airs on Mondays at 9pm on Watch.
> The 2012-13 season TV scorecard: What's axed? What's renewed?
Watch a clip from the next Grimm below: