Thomsen scored the only Danish goal in the 1–1 draw against Iceland which qualified Denmark U-21 for the 2015 UEFA European Under-21 Championship. He was selected for the squad for the tournament and played the entire match in the opening win against the Czech Republic. He was in the line-up in the other two group stage-matches against Germany and Serbia. Thomsen was also chosen to the line-up in the semi final, where Denmark was defeated by Sweden.