Stewart - who stars in The Dark Knight Rises - will appear in the NBC drama's second season, according to TVLine.
His character Bill - to debut in episode seven - has the ability to transform into a creature named a Drang-Zorn, which is similar in appearance to a badger.
Stewart's past television roles include a recurring part on CBS drama Criminal Minds and a stint on ABC's short-lived series No Ordinary Family. His film credits include 2008's The Curious Case of Benjamin Button and 2009's Law Abiding Citizen.
Other stars signed up to appear in season two of Grimm include Maddie Hasson (The Finder), James Frain (True Blood, The Cape) and Mark Pellegrino (Lost, Supernatural).
Grimm returns to NBC on Monday, August 13 at 10/9c. In the UK, the series airs on Watch.
> Grimm exec teases season two, new enemies
Watch a video recap of Grimm season one below: