In June 2012, Boasson Hagen won the Norwegian National Road Race Championships to take the national jersey for the first time; he did however lose his time trial national championship jersey to 's Reidar Borgersen. Boasson Hagen was selected in the Sky squad for the Tour de France. He finished fifth in the opening prologue in Liège, then took third on stage one, after bridging across to Peter Sagan and Cancellara in the closing stages, but being unable to beat them both in the sprint. He also finished second to Sagan on stage three, another hilltop finish. At the end of August, Boasson Hagen took the victory in the one day World Tour race the GP Ouest-France. He broke away from the bunch with a little more than two kilometers to go, caught and dropped the remaining man in front, Rui Costa () on the last uphill section and descended to the finish solo. He denied the sprinters in the eighty-man chasing group a chance at victory by holding on to a five-second advantage.