"This is huge. I've said all along I wanted to be a Bruin as long as possible," Thornton said. "I love the city and I love being here. I'm really happy to be back and I'm glad we could make it happen."
The 32-year-old Thornton had a goal, nine assists and a career-high 141 penalty minutes in 74 games in 2009-10, his third season with the Bruins. He could have become a free agent on July 1.
Thornton's 21 fighting majors led the team and tied for fifth in the NHL.
"Shawn has been a terrific soldier for us in the time that he's been here and he always amongst the leaders in fisticuffs, I'll use fisticuffs instead of fighting because it's an art for Thorny," general manager Peter Chiarelli said. "He knows his role and he has an offensive side that comes out. He has tremendous character and is good in the room. He wants to be in Boston, so it was a good decision we made. It was an easy decision we made and we're happy to have him back."
Chiarelli also talked up Thornton's role as a leader on the Bruins.
"It's important to have any good veteran to help out with the younger guys," Chiarelli said. "His career path has not been the easiest and he commands a level of respect from that reason. He's put in his time and he's put in his work and he's been rewarded for it.
"Thorny is one of the guys we want to have around and he's a good person. He works hard and comes to work every day and sets a very good example."
Information from ESPNBoston.com's Joe McDonald, ESPN.com's Pierre LeBrun and The Associated Press was used in this report.