During his first North American professional season and while eligible for his final junior tournament, Rantanen was loaned by the Colorado Avalanche to Captain the Finnish junior team as they hosted the 2016 World Junior Championships. While adding a stabilising veteran presence, Rantanen played a supporting role throughout the round-robin stage before stepping up his production in the Semi-final against Sweden and scoring in the Final against Russia to help Finland claim the gold medal and cap his junior career.