Spezza had been on the trading block for weeks after requesting a deal from Ottawa, with both the Stars and St. Louis Blues emerging as two of the most aggressive suitors in acquiring the former Senators captain.
Stars general manager Jim Nill won that sweepstakes, adding one of the top centers available to a lineup that already boasts young pivot Tyler Seguin and star forward Jamie Benn.
The team also signed free agent forward Ales Hemsky on Tuesday to a contract reportedly worth $12 million over three years.
Hemsky and Spezza were briefly teammates in Ottawa last season.
The 31-year-old Spezza is coming off a 23-goal, 66-point season for the Senators. He has recorded four seasons of 30 goals or more in his 11 years with Ottawa.
Spezza has one year remaining on a seven-year deal that pays him $7 million annually.
He was one of the top three centers that became available on the trading block leading up to what has already been a wild offseason.
Ryan Kesler was shipped from the Vancouver Canucks to Anaheim Ducks last week, and the San Jose Sharks are dangling Joe Thornton, though he maintains that he would like to remain a member of the Sharks and has a no-move clause that allows him to stay put.
The deal sending Spezza to the Stars comes on the heels of the former first-round pick (second overall in 2001) vetoing a trade to the Nashville Predators last weekend. Senators GM Bryan Murray had a deal in place that would have sent Spezza to the offensively challenged Predators, but he exercised his no-trade clause and nixed the deal, a move that seemed to irritate both Murray and Predators GM David Poile.
After the deal fell through, Poile said he didn't want to pitch Spezza on coming to Nashville if he wasn't interested in playing for the organization.
Now, Spezza has found a home in the Western Conference with a Stars team that surprised many with an encouraging turnaround this season. Dallas lost in the Western Conference quarterfinals in a nasty first-round match against the Ducks.
The 30-year-old Hemsky, meanwhile, spent his first 10 seasons with Edmonton before a midseason trade to the Senators in March. The Czech winger has 146 goals and 348 assists in 672 games.
The Stars also signed goalie Anders Lindback to a one-year contract. The 26-year-old appeared in 23 regular-season games and all four playoff games for Tampa Bay last season. The four-year veteran has a .904 save percentage and 2.74 goals-against average for his career.
ESPN.com's Pierre LeBrun and The Associated Press contributed to this report.