The Rangers will also send a first-round draft pick in 2015 and a conditional second-round choice in 2014 to the Lightning in exchange for the reigning NHL scoring champ. The second-round pick will become a first-round choice if the Rangers reach the conference finals this season.
Callahan is set to become an unrestricted free agent at the conclusion of the season. If Callahan re-signs with Tampa Bay, the Rangers will receive Tampa Bay's second-round pick in the 2015 draft and the Lightning will acquire New York's seventh-round pick in 2015.
St. Louis, who led the league with 60 points in the shortened 48-game season in 2013, was the leading scorer for the Lightning this season with 29 goals and 32 assists for 61 points in 62 games.
The 38-year-old veteran joined the Lightning as a free agent in July 2000 after spending his first two seasons in Calgary. He helped Tampa Bay to its lone Stanley Cup title in the spring of 2004, when he was also the Hart Trophy winner as the league MVP, and will be reunited with former Lightning teammate Brad Richards in New York.
St. Louis has 369 goals and 604 assists for 973 points in 1,041 career NHL games. He has one year remaining on his contract and requested a trade after general manager Steve Yzerman initially left him off the Canadian Olympic roster. Yzerman, also the GM for Team Canada, eventually added St. Louis as an injury replacement for Steve Stamkos.
"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," said Yzerman. "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."
Callahan and the Rangers were trying to work out a new deal. He had spent his entire eight-year career with New York and has 132 goals with 122 assists for 254 points in 450 NHL games.
"The Lightning are very excited to have Ryan Callahan join the organization and help us in our quest to make the playoffs," said Yzerman. "Ryan is an outstanding player and leader who we look forward to seeing on the ice (Thursday) night."
The 28-year-old Rochester, N.Y. native has 11 goals and 14 assists for 24 points in 45 games this season.