The surprising move was announced just hours before the afternoon deadline and shortly before the Rangers hosted the Toronto Maple Leafs. St. Louis arrived in time to make his debut with New York, but the Rangers lost 3-2 in overtime.
''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 and 11 seconds of ice time Wednesday night. ''This is a chance to play the game in one of the biggest markets, and I know what comes with it.''
The Rangers had been trying to sign Callahan, who can become an unrestricted free agent this summer, but weren't able to reach an agreement with the gritty forward. They sent him packing instead of risking losing him for nothing.
''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. 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.''
New York dealt Callahan, a second-round pick in this year's draft, and a first-round pick in next year's draft to Tampa Bay for St. Louis, a disgruntled two-time NHL scoring champion.
The second-round selection this year would become a first-rounder if the Rangers reach the Eastern Conference finals this season. If Callahan re-signs with the Lightning, the teams will then trade picks in next year's draft. New York would acquire the Lightning's second-round choice, and Tampa Bay would get the Rangers' seventh-round selection.
Callahan was reportedly seeking a six-year deal worth $6.5 million per season. It is believed the Rangers were willing to agree to a six-year term, but not pay more than $6 million a season.
''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.''
The 38-year-old St. Louis, who led the NHL in the lockout-shortened 2013 season with 60 points, had been seeking a trade after initially being left off Canada's Olympic roster by Lightning general manager Steve Yzerman, who served as Canada's executive director. St. Louis was added when Lightning teammate Steven Stamkos was unable to play because of injury.
Callahan, a Rochester, N.Y., native, was a member of the U.S. Olympic team at the Sochi Games. 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 the Lightning and Rangers are in Eastern Conference playoff positions.
''We're adding a player that's got more of an offensive capability,'' Rangers coach Alain Vigneault said. ''We're losing Cally's grit and fire on the ice, but we're adding some other really important parts to our team. I really believe this makes us a better team.''
St. Louis waived a no-trade clause to come to the Rangers. The right wing has one year left on his contract. He will earn $5 million next season and be a $5.625 million salary-cap charge.
''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 a statement. ''He has been one of the greatest players in the organization's history, but in the end we honored his request.''
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.''
Rangers general manager Glen Sather said the club and Callahan had narrowed the gap on the money, but a no-trade clause became a sticking point.
''The no-trade was the one thing that really bothered me in the end,'' Sather said. ''You give too much away when you give that no-trade contract up, it really ties your hands. I know it's nice for the player to have security, but no-trade, no-yell-at is a tough deal.''
This trade reunites St. Louis with forward Brad Richards, who was his high-scoring teammate with Tampa Bay before the Lightning traded him to Dallas in February 2008. Richards signed a free-agent deal with the Rangers in July 2011.
They are already linemates again with the Rangers.
''It was the first time I had a good friend coming and a good friend leaving, which is bizarre,'' Richards said. ''You are not really happy or sad.''
St. Louis was the Hart Trophy winner as NHL MVP in the 2003-04 season, and he has captured the Lady Byng Trophy as the league's most gentlemanly player three times.
In 62 games this season, St. Louis has 29 goals and 32 assists with a plus-12 rating. He is fifth in the league in goals and eighth in points. Last season he became the oldest player to win the scoring title and joined Mario Lemieux and Gordie Howe as the only players to post at least 1.25 points per game at 37 or older.
St. Louis, a six-time All-Star, has 369 goals and 604 assists in 1,041 career games over 15 seasons with Tampa Bay and Calgary.
Callahan, who will turn 29 later this month, has 11 goals and 14 assists in 45 games this season. He has 132 goals and 254 points in 450 career NHL games over eight seasons.
He became Rangers captain before the 2011-12 season - the fifth youngest in team history at age 26.
Callahan took part in the Rangers' morning skate Wednesday, but didn't speak to reporters afterward. He is expected to make his Lightning debut on Thursday at home against Buffalo.
Just before the deadline, the Rangers also acquired 28-year-old defenseman Raphael Diaz from Vancouver for a fifth-round pick in 2015.
Diaz has one goal and 12 assists in 52 games this season with the Canucks and Montreal Canadiens. He had one goal and one assist in six games after being acquired by Vancouver on Feb. 3. In 134 career games over three NHL seasons, Diaz has five goals and 38 assists.