Stewart has represented Canada internationally in two World Junior Championships, in the 2003 IIHF World U18 Championships, and twice in the ADT Canada-Russia Challenge. He was the leading goal scorer for Team Canada when they won the World U18 Championships. In the 2004 World Junior Ice Hockey Championships, in which Canada won a silver medal, Stewart tied for the tournament points lead. In the 2005 World Junior Ice Hockey Championships, he helped Team Canada win a gold medal. Representing the OHL at the Canada-Russia Challenge, Stewart helped his team go unbeaten in both tournaments (4–0), helping Canada to win the Challenge two years running.