The Rangers sent a first-round pick in 2015 and a second-round pick in 2014 to Tampa Bay in the deal, which was completed in the final hours before Wednesday's 3 p.m. ET trade deadline.
There are two picks-related conditions in the trade. If the Rangers reach the Eastern Conference finals this season, the 2014 pick headed to Tampa Bay would become a first-round selection. If Callahan, a pending unrestricted free agent, re-signs with the Lightning, New York would receive a second-round pick in 2015 and Tampa Bay would get an additional seventh-round pick in 2015.
St. Louis made his Rangers debut Wednesday night against the Toronto Maple Leafs at Madison Square Garden. The Rangers lost 3-2 in overtime, but St. Louis got a nice ovation when he hopped over the boards for his first shift 38 seconds in, and he created a surge of excitement when he fired a hard shot on goal from the top edge of the left circle.
"I know this is going to be a challenge for me, but I love challenges and I like to rise to the occasion," St. Louis said after recording three shots in 20 minutes, 11 seconds of ice time. "This is a chance to play the game in one of the biggest markets, and I know what comes with it."
St. Louis requested a trade out of Tampa Bay after he was initially snubbed by general manager Steve Yzerman -- also the GM of Team Canada -- when his boss named the 2014 Olympic squad. St. Louis later was added as an injury replacement for the Lightning's Steven Stamkos and won a gold medal with Team Canada, but that apparently did not change the situation.
When asked if St. Louis had a hand in making the trade happen, Yzerman told reporters on Wednesday afternoon the two had "multiple" discussions about St. Louis' future with the franchise. However, Yzerman mostly deferred to St. Louis saying, "I'm not going to stand here and judge him."
St. Louis, who was named Tampa Bay's captain prior to this season, had a no-trade clause in his contract and reportedly told Yzerman that New York was his preferred destination. Yzerman acknowledged St. Louis' trade request in a written statement Wednesday.
"We'd like to thank Marty for everything he has done on and off the ice during his outstanding 13-year career in Tampa Bay," Yzerman said in the statement. "He has been one of the greatest players in the organization's history, but in the end we honored his request today. We wish him and his family the best of luck as he continues his career in New York."
St. Louis issued an open thank you letter to Lightning fans, in which he declined to explain his reasons for wanting a trade.
"Today is a bittersweet day for me," he wrote. "I am sad that this chapter of my career is over. I have had 14 wonderful years in Tampa and have cherished being a member of the Tampa Bay Lightning.
"In the end this is a decision for my family. I respect the fact that many of you do not agree with my decision and are angry with it. All I really can say is that I am sorry and I am very appreciative of the support you have shown me through the years."
Callahan heads to Tampa Bay after a protracted contract negotiation with New York failed to yield an extension despite the sides being very close on a new deal.
"It's still tough," Callahan told Canada's TSN. "I knew it was an option that was going to happen if I didn't re-sign with New York. No matter how prepared you are for it or knowing it's going to come, when you hear that initial, 'You've been traded,' it's definitely a shock."
Yzerman said he expects Callahan to make his Lightning debut Thursday against the Buffalo Sabres.
"The Lightning are very excited to have Ryan Callahan join the organization and help us in our quest to make the playoffs," Yzerman said in the statement. "Ryan is an outstanding player and leader who we look forward to seeing on the ice tomorrow night."
Asked if the Lightning are a better team following the trade, Yzerman told reporters, "Are we better? I don't know. We're different. We'll see."
According to one source, Callahan and New York were within "spitting distance" as of Wednesday morning, when the Rangers appeared to refocus their efforts toward acquiring St. Louis, the reigning Art Ross Trophy winner.
Callahan had been seeking a six-year deal worth between $6.25 million and $6.5 million annually, a source confirmed Wednesday to ESPN.com. The Rangers were willing to offer him a six-year deal at $6 million per year.
"I truly thought we'd work something out," Callahan said. "I said all along going through this that I wanted to stay there and get a deal done, and that was the truth. But it's part of the business that, unfortunately, you have to deal with."
On Wednesday, Rangers GM Glen Sather cited a no-trade clause and the length of a contract as things that contributed to the end of the Callahan contract talks.
"We were getting closer on the money. But in this league, eight years is an awful lot of money for Ryan Callahan, or anybody else, as a matter of fact," Sather said. "Unless you're a first All-Star team player that's won Stanley Cups, and there's lots of things that fall into that.
"The no-trade is the one thing that really bothered me. ... You give too much away when you give that no-trade contract. Because it really ties your hands."
Callahan said he plans to wait and get acclimated in Florida before deciding whether to open contract talks with the Lightning.
"I'm extremely excited. I'm going to a great organization, great owners, great GM," he said. "I've heard nothing but good things from players that have played there, and they're in the playoff hunt, too, which is big."
Both Sather and Rangers coach Alain Vigneault noted that trading the team's captain could certainly affect the team's chemistry but Vigneault seemed certain St. Louis' talent would help smooth things over.
"There's no doubt taking a player of that magnitude from our dressing room is going to have an impact," Vigneault explained. "That being said, we're also adding a player that's got great presence, that's won a [Stanley] Cup, that's just come back from winning a gold medal for Canada.
"We're adding a player that's got more of an offensive capability ... I really believe this makes us a better team."
St. Louis, 38, has one year left on his contract. He will earn $5 million next season and be a $5.625 million salary-cap charge. He entered Wednesday with 29 goals and 61 points -- the eighth-highest total in the NHL -- in 62 games. Prior to the trade, Rangers center Brian Boyle referred to St. Louis as a "great player."
"I've met Marty a few times," Boyle said. "I know what kind of player he is, what kind of guy he is. He's a great player."
ESPN.com's Pierre LeBrun, ESPNNewYork.com's Johnette Howard and The Associated Press contributed to this report.