"I'm excited to be part of a great organization," Seidenberg said. "I think Boston's a great town, a great hockey town, the guys are great and think the future for the organization will be really good."
General manager Peter Chiarelli announced the move on Saturday.
Seidenberg's deal is worth $13 million over four years with an average annual salary cap hit of $3.25 million, a source told ESPN.com's Pierre LeBrun.
Seidenberg has played in 374 regular season games and has 18 goals and 98 assists in seven NHL seasons.
Seidenberg was traded from Florida to Boston with Matt Bartkowski at the trade deadline.
He played 17 games for the Bruins and had two goals and seven assists. Seidenberg was also paired with Norris Trophy-winner Zdeno Chara, forming a dynamic defensive duo for Boston.
"Dennis is more than that," Chiarelli said when asked about Seidenberg's shutdown ability. "He has the ability to do that and more, which is what made him attractive to us. Shutdown is strong, defensively sound, moves the puck well from a good defensive position. But I think you've also seen his ability to make a very good pass, he's got a very good one-timer, he sees the ice well, he pinches in the offensive zone. So he has the ability to play that role and that's first and foremost why we got him. We knew there were other facets of his game."
Added Seidenberg when asked about his own improved game: "Everybody's striving for perfection. I want to be a better consistent player, to work well every game. I think that's the hardest thing to do as a player. And also just my game ... maybe just be more consistent, and everything else comes with that."
Seidenberg missed the final four games of the regular season and the playoffs after suffering a lacerated tendon in his forearm on April 3. He had surgery three days later to fix the injury.
Information from ESPNBoston.com's Matt Kalman and The Associated Press contributed to this report.