The $58-million deal begins with the 2014-15 season and runs through 2021-22.
The contract averages $7.25 million per season.
A nominee for the NHL's Norris Trophy this season, Letang led all NHL defenceman in assists (33) and was tied for the lead in points (38).
The six-foot, 201-pound Montreal native established career highs during the post-season with three goals and 13 assists.
In 385 career games, the 26-year-old Letang has 44 goals and 165 assists.