He represented Norway at the FIS Nordic World Ski Championships 2015 in Falun. Known for his classic technique, Tønseth proved to be equally adept at freestyle, proving this in the 30 km where he was the sole Norwegian with a shot at a medal. He narrowly lost the final sprint to Canadian Alex Harvey, and came fourth. He skied the second leg of the relay, earning him a gold medal after Northug's strong finish.