The 28-year-old "UnReal" alum will play Samwell's brother, Dickon, said The Hollywood Reporter.
In his novel, George R R Martin has told very less about the character Stroma will play, except that he will be young and more physically capable his brother.
Stroma joins Ian McShane, "The Borgias" alum Pilou Asbaek and Max von Sydow as the new faces for season six.
The new season of Game of Thrones is expected to premiere sometime in 2016 on HBO.