At age 16, Dahlin was the youngest player to ever dress for Sweden at the IIHF World U20 Championship, and the youngest player overall in the 2017 World Junior Ice Hockey Championships. Dahlin competed in the 2018 World Junior Ice Hockey Championships for Sweden, recording six assists and helping lead the team to win silver. He was named the tournament's top defenceman after scoring the second-most points amongst tournament defencemen. After Dahlin and several other teammates removed their silver medals after receiving them during the medal ceremony, he was suspended for the first two games of the 2019 World Junior Ice Hockey Championships.