Seguin was sent to the Stars on Thursday in a seven-player deal also highlighted by steady forward Loui Eriksson joining the Eastern Conference-champion Bruins.
Dallas got forward Rich Peverley and defenceman Ryan Button, while the Bruins acquired forwards Matt Fraser and Reilly Smith, and defenceman Joe Morrow.
It was the first major move for new Stars general manager Jim Nill, who addressed the team's need at centre with Seguin, the No. 2 pick in 2010, and Peverley. The trade will allow Dallas' leading scorer, Jamie Benn, to move to his more natural spot at wing.
"Tyler is a dynamic player that will be a part of our core group for a long time to come," Nill said. "A player at his age, position and talent level are extremely difficult to acquire and we're thrilled to bring him into our organization."
Seguin won the Stanley Cup with the Bruins as a rookie and was their leading scorer and an All-Star a year later. He dropped to a tie for third on the team in scoring during this past season's run to the Stanley Cup finals. Boston lost to Chicago in six games.
At the draft last weekend, Bruins general manager Peter Chiarelli essentially said that Seguin wasn't focused enough on hockey, although he made it sound like the 21-year-old would be back by saying he was expecting "big things from him."
Instead, the Bruins have added Dallas' second-leading scorer in Erikkson, a 2011 All-Star who turns 28 this month. He and Ray Whitney were second to Benn with 29 points.
Seguin leads all players from the 2010 draft class in games played (203) and is third in goals (56), assists (65) and points (121). Peverley is viewed as a No. 2 centre who is strong in the faceoff circle, while Button will join a veteran blue line that recently added 39-year-old Sergei Gonchar on a two-year, $10 million contract.
Smith had nine points in 37 games for the Stars while shuffling between Dallas and the Texas Stars of the AHL. Fraser appeared in nine games for Dallas, while Morrow has yet to make his NHL debut.