Seidenberg, 32, is in the final year of a four-year, $13 million contract with the Bruins.
"It makes me really happy, and on top of it we won the game," Seidenberg said after Boston's 3-1 win against the Lightning in the season opener Thursday night at TD Garden.
The Bruins acquired Seidenberg via trade with the Florida Panthers on March 3, 2010, and he's been a staple on Boston's blue line since his arrival.
"He's been one of our core guys since we got him," Bruins general manager Peter Chiarelli said. "You hear me talk often about character guys; he's definitely that on and off the ice. He plays our style -- heavy game, heavy game that we like. He's been a warrior for us. There's a lot of things that he has that are compatible with the way we want to play. He's 32 but we feel he's in very good condition and takes good care of himself. Frankly, he wanted to stay here so it was easier to get it done for that reason."
Seidenberg probably left a lot of money on the table by accepting and signing this particular deal before hitting the free-agent market after this season, but the stability and the winning mentality in Boston were the reasons he wanted to remain in a Bruins sweater.
"It was very important," he said. "I have a lot of fun playing here. The coaches have trust in me and that's the most important thing. I mean, if I go somewhere else for a lot of money and things don't work out that way, it's always -- I think the on-ice situation is the most important part. We have a good team here and in the future I think we're going to be good for a lot of years."
There's a no-trade clause for the first 2½ years, then it's a partial no-trade clause for the remainder of the contract.
"If they want to trade me, I don't want to be here, right? It's good for both sides," Seidenberg said. "It's a deal that works out for both sides. I want to be here and they want me here and both sides gave a little bit and I'm really happy to be here for another five years.
Bruins goaltender Tuukka Rask, who signed an eight-year deal worth $56 million this summer, also wanted to see one of his best defenseman remain in Boston for the long term.
"He's a solid D-man," Rask said. "He's got that offensive ability, too. He can do whatever out there. He's an all-around D-man. We've played together for a couple of years now, so I think it's important to keep that defensive core there because it helps my job."