In July 2017, Barrett made headlines with a 38-point, 13-rebound, and five-assist showing as he guided Canada's under-19 squad to a 99–87 semifinal win over the US at the FIBA Under-19 Basketball World Cup. At the time, Northpolehoops.com described this performance as Barrett's "biggest game of his career so far". He subsequently led Canada to the title, tallying game-highs of 18 points and 12 rebounds in the championship game against Italy, and was named to the tournament's All-Star Five and also the tournament Most Valuable Player. Averaging 21.6 points a game, he was also the leading scorer of the tournament.