Karlsson was selected as the tournament's best defenceman at the 2008 IIHF World U18 Championships. He was the tournament's &lt;a href="Plus-minus%20%28ice%20hockey%29"&gt;plus/minus&lt;/a&gt; leader with a plus eight rating. He led team Sweden in assists, finishing third overall in the tournament with seven in six games, which also tied him for first place in defencemen scoring. During the 2008 U20 4-Nations tournament which Sweden won, Karlsson scored one goal and one assist which tied him with David Rundblad, Viktor Ekbom and Tim Erixon as Sweden's defenceman scoring leader. At the 2009 World Junior Ice Hockey Championships, Karlsson was selected to the all-star team, and as the tournament's best defenceman. With two goals and seven assists in six games, he led all Swedish players in points, and was tied for the tournament lead among defencemen.