On April 29, 2019, Marchessault was selected to his maiden international tournament after he was named to the Team Canada roster for the 2019 IIHF World Championship, held in Slovakia. Placed on Canada's first line he helped Canada progress through to the playoff rounds before losing the final to Finland to finish with the Silver Medal on May 26, 2019. Marchessault finished the tournament contributing with 3 goals and 10 points in 10 games.