Kassian debuted internationally with Hockey Canada at the 2008 World U-17 Hockey Challenge, competing for Team Ontario. He recorded two goals and six points over six games as Ontario won the gold medal. The following year, he moved on to the under-18 level with Canada at the 2009 IIHF World U18 Championships. With two goals and five points over six games, he helped Canada to a fourth-place finish. Two years later, Kassian joined Canada's under-20 team for the 2011 World Junior Championships in Buffalo, New York. During the tournament, he received a two-game suspension for a hit against Czech defenceman Petr Senkerik, making contact with Senkerik's chin with his shoulder. Kassian returned for the medal round and helped Canada to the gold medal game, where they lost 5-3 to Russia. He finished the tournament with two goals and an assist over five games.