The Three-Eyed Raven is Bran Stark's mysterious tutor, who usually appears in his dreams.
It was earlier played by Struan Rodger. The character was briefly seen in the fourth season finale when Bran finally met his mentor, reported Aceshowbiz.
The recasting comes after it was confirmed that Bran would return in season six, after being absent in the whole season five.
Executive producer David Benioff hinted that the show would pick up Bran's storyline after he has been trained by the Three-Eyed Raven for a while.
Sydow, 86, joins Ian McShane, who was recently added to the cast of "Game of Thrones" to play a mystery role.