Following his success in the sixth stage in Lisieux, and fellow countryman Thor Hushovd's trailblazing rides during stages 13 and 16, Boasson-Hagen made sure Norway enjoyed another day of celebration thanks to his astute tactics.
The 24-year-old took part in the decisive 14-man breakaway and went on his own in the climb overlooking Pinerolo to cross the line first. Wednesday's ride helped to heal some sore wounds for Boasson-Hagen as he had been beaten on the line by Hushovd just 24 hours earlier.
Dutchman Bauke Mollema was second, 40 seconds behind, and Frenchman Sandy Casar finished third, 50 seconds adrift.
"It's a great day, I really wanted to win this stage after finishing second yesterday, which was very frustrating," said Team Sky rider Boasson-Hagen.
France's Thomas Voeckler retained the Tour leader's yellow jersey but lost 27 seconds in the tricky descent towards the finish line when he missed a turn and ended up in a private car park.
Overall, Voeckler now leads Australia's Cadel Evans by one minute and 18 seconds and Luxembourg's Frank Schleck by 1:22.
Three-times Tour champion Alberto Contador again confirmed he was back in great shape after his attacks in the previous stage when he broke from the favourites peloton in the descent with compatriot Samuel Sanchez.
The two Spaniards held a 20-second lead over the other Tour contenders before being reeled in the final stretch.
Thursday's 200.5-kms 18th stage to the Galibier is expected to be one of the most demanding in this Tour, with three classic ascents, Col d'Agnel, Izoard and the final climb.
