Several teams, including the Anaheim Ducks, had expressed interest in Girardi, had the Rangers been unable to sign him to an extension.
TFP reported earlier on Friday that the two sides had been making progress on contract negotiations and the Rangers were stuck on wanting a five-year deal, but they ultimately accepted a sixth year on Girardi's deal.
Girardi's contract includes a full no-movement clause in the first three-years of his deal and a limited no-trade clause in the final three-years.
In 60 games with the Rangers so far this season, Girardi has picked up three goals and 17 assists for 20 points, along with 14 penalty minutes.
The Rangers will now focus on trying to re-sign captain Ryan Callahan, though many believe he will be traded prior to the March 5 trade deadline.