Callahan wasn't able to come to terms on a new contract with the Rangers, prompting the trade.
If the Rangers make the Eastern Conference Finals this season, the second-round pick becomes a 2014 first-round draft pick.
If the Lightning re-signs St. Louis, the Rangers will get a 2015 second-round draft pick and the Lightning get a 2015 seventh-round pick.
In 62 games with the Lightning this season, St. Louis registered 29 goals and 32 assists for 61 points, along with a plus-12 rating.
The native of Laval, Quebec, has played in 1,041 career NHL regular-season games with the Calgary Flames and Lightning, and has accumulated 369 goals and 604 assists for 973 points.
Callahan, 28, has tallied 11 goals and 14 assists for 25 points in 45 games with the Rangers this season.
The native of Rochester, New York, has collected 132 goals and 122 assists for 254 points in 450 career NHL regular-season games, all with the Rangers.
He was originally drafted by the Rangers in the fourth-round (127th overall) of the 2004 NHL Entry Draft.