Dion Phaneuf, left, and Jamal Mayers will be trading jerseys, with Phaneuf headed for Toronto and Mayers bound for Calgary.
The Leafs also acquired veteran goalie Jean-Sebastien Giguere from the Anaheim Ducks for Jason Blake and Vesa Toskala.
The Leafs acquired Phaneuf, right wing Fredrik Sjostrom and defenseman Keith Aulie in exchange for defenseman Ian White, center Matt Stajan, left wing Niklas Hagman and right wing Jamal Mayers.
"Of course the centerpiece of the deal is Dion Phaneuf," Leafs GM Brian Burke said at a news conference Sunday. "A couple of words come to mind with him, like 'elite player.' He's a warrior, he's got a cannon for a shot and he plays the game hard. I want players that play the game hard. He makes our power play better, he's a big hitter and a quality human being. We're thrilled to have him."
The shakeup comes after Toronto's 5-3 loss to the Vancouver Canucks on Saturday night to extend its winless streak to six games. The Leafs (17-28-11, 45 points) are tied with Carolina for last in the Eastern Conference.
"We've got to create tension where there's enough talent on the roster and a high enough battle level that the coach has decisions to make," Burke said. "Right now he doesn't have decisions to make. It's these guys who got to go on the ice. We're trying to change that. Today is an important part of changing that.
"We're still open for business, we're not done," he added.
Phaneuf had 10 goals, 12 assists and 49 penalty minutes as of Sunday. In 378 career games over five seasons he has 75 goals, 153 assists and 522 penalty minutes, all with the Flames.
"It was definitely a shock, but I'm very excited about going to Toronto and being a Maple Leaf," Phaneuf said.
"It's part of the business." he added. "It's a cliche. If Wayne Gretzky can get traded anyone can get traded. I was very surprised, but on the other hand I'm very excited to be going to the biggest hockey market in the world."
The Leafs gave up two of their top scorers in Stajan (16 goals, 25 assists) and Hagman (20 goals, 13 assists) to obtain Phaneuf.
"We're just going to have to find other ways to manufacture offense," Toronto coach Ron Wilson said.
"In one fell swoop, we've made ourself the youngest team in the league," he added. "Instead of looking back, we're looking forward."
Burke said the Flames didn't want to deal Phaneuf earlier in the season.
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"We asked about him earlier in the year and the answer was no," Burke said.
But the Flames had a nine-game winless streak and Calgary GM Darryl Sutter was clearly feeling the pressure to make a big move.
Phaneuf has four more years left on his deal after this season at an average of $6.5 million per year.
White will be a restricted free agent July 1, while Stajan will be an unrestricted free agent.
The trade of Giguere made sense for the Ducks, as they signed goalie Jonas Hiller to a four-year contract extension on Saturday.
Giguere had to waive his no-trade clause for the deal to go through, "but he wanted to come here," Burke said. "When I spoke to him, he was excited."
"Obviously I have a history with this player, won a championship with him in Anaheim," Burke said. "But more importantly he has a history with our goalie coach."
Leafs goalie coach Francois Allaire was Giguere's goalie coach in Anaheim until coming to Toronto last summer.
Blake has two more years left after this season at $3 million per year, although his annual salary cap hit is $4 million. Toskala is an unrestricted free agent as of July 1.
The acquisition of Giguere will allow the Leafs to give Swedish rookie Jonas Gustavsson more time to learn.
Toskala has a 3.66 goals-against average over 26 games while Gustavsson -- who twice has been treated for a heart problem this year -- has a 3.03 GAA in his 31 appearances.
Sjostrom, taken 11th overall in the 2001 draft by Phoenix, has one goal and five assists in 56 games this season.
White had nine goals and 17 assists in 56 games and Mayers two goals and six assists in 48 games.
Blake had 10 goals and 16 assists in 56 games.
Giguere has a 4-8-5 record with a 3.14 GAA and a .900 save percentage this season. Hiller is 21-15-2 with a 2.75 GAA and a .920 save percentage.
Giguere was drafted 13th overall by the Hartford Whalers in the 1995 draft. He played in the Flames organization for four seasons before joining the Ducks in 2000.
He captured the 2003 Conn Smythe Trophy before winning the Stanley Cup with the Ducks in 2007.
Information from ESPN.com's Pierre LeBrun and The Associated Press was used in this report.