The actor has signed up to play Jaquen H'Ghar in future episodes of the show, fansite Westeros reports.
Jaquen is a Lorathi prisoner who is recruited to the Night's Watch by Yoren (Francis Magee).
Most of Wlaschiha's previous work is from Germany, although he has appeared in the British shows The Deep and The Sarah Jane Adventures. He also starred in the one-off Matt Smith drama Christopher And His Kind.
Wlaschiha is the latest actor to sign up for Game of Thrones after Nonso Anozie and Gemma Whelan.
Other stars who have joined the cast for the second season include Liam Cunningham, Carice van Houten, Stephen Dillane, Natalie Dormer and Gwendoline Christie.
Game of Thrones is expected to return for its new season in the spring.
Watch Emilia Clarke chatting to Digital Spy about the sex and nudity in Game of Thrones below: