Sources told ESPN NFL Insider Adam Schefter that the extension, which runs through the 2022 season, is worth $61 million.
"This is one of the best organizations to be a part of, especially for a guy like me," Tuitt told Steelers.com. "I (came in) as a young guy, super raw. I have some great coaches who have helped me and I have seen my game pick up, my confidence level rise, my leadership ability rise. Being a part of an organization like this, I can see why a lot of great players come from here, a lot of Hall of Fame players come from here."
The team worked all week to lock up a key piece to their defensive front seven. Tuitt, a second-round pick in 2014, has 108 tackles and 11.5 sacks in three seasons.
The Steelers -- who don't negotiate during the season -- are the only NFL team with two 3-4 defensive ends playing on contracts in excess of $50 million. Cameron Heyward signed a six-year, $59 million extension prior to the 2015 season.
Tuitt, who was scheduled to be a free agent in 2018, is a versatile end who plays inside or out, base or nickel, teaming with Heyward as interchangeable parts.
The Steelers open the season against the Cleveland Browns on Sunday.