Puljujärvi played for Finland at the 2016 World Junior Championships, where they won gold on 5 January 2016. In seven games, he scored 5 goals and 12 assists for 17 points, placing him second in all-time points for an under-18 player behind only Jaromír Jágr, and tying Wayne Gretzky and Eric Lindros. He was subsequently declared Best Forward and Most Valuable Player of the tournament. Later that year, Puljujärvi was also a part of the national team at the 2016 World U18 Championships, winning another gold medal, becoming the first and so far the only European player to win a U20 and U18 gold medal in the same year.