The True Blood actor, who played Franklin on the third season of the HBO series, will have a recurring role on the second season of the NBC show, according to Zap2it.
Frain will be playing Eric Renard, the brother of Captain Sean Renard (Sasha Roiz).
The actor previously appeared on The Tudors, as well as the short-lived series The Cape.
Grimm actress Bree Turner was promoted to a series regular in April.
The series was renewed for a second season in March. It was the first NBC show to be picked up for another season.
Grimm will return in the fall at Fridays on 9pm.