Seguin beat Select goalie Stephen Murphy on a power play in the second period to tie the game at 1. He tallied his second goal on a penalty shot in the third period.
"We got a power-play goal and our line went out and just worked hard and got us a couple to get us the lead," Thornton said. "It was nice that we had the momentum going into the third period."
"These are tough games for us," Bruins coach Claude Julien said. "We had nothing to win in the game but a lot to lose, and Belfast had nothing to lose and everything to win."
NHL.com contributed to this report.