The contract for the 26-year-old blueliner replaces a four-year deal that had been set to run through the 2013-14 season.
Terms of the deal weren't announced, but the Pittsburgh Post-Gazette reported it was worth $58 million, with an annual salary cap hit of $7.25 million.
Letang tied for the NHL lead for defensemen with 38 points last season and was the tops in the league in assists. He added 13 assists and 16 points over 15 post-season games for the Penguins, who reached the Eastern Conference finals.
Letang has 44 goals and 165 assists in 385 career regular-season games over six seasons with Pittsburgh.