The center, who could have become an unrestricted free agent after this season, had 54 points (17 goals, 37 assists) in 72 games for the Blues last season and 12 points (five goals, seven assists) in 26 Stanley Cup Playoff games on the way to St. Louis' first championship.
"The organization, management, coaches have shown faith in me moving forward here, so obviously when they're going to do that, you want to buy in, and when you see how good the team is here, how good we were last year, how good we can be, I think it only makes you want to stay," Schenn said.
"Obviously over the period of the contract, the term, the team is going to change, but the one thing that doesn't is the city, and it's a great city to live in, great organization to play for. I'm looking forward to it."
Schenn, who is in his third season with the Blues, has 124 points (45 goals, 79 assists) in 155 games with St. Louis. He was acquired in a trade with the Philadelphia Flyers on June 23, 2017.
"Certainly getting 'Schenner' signed was a priority for us over the summer, and working with (agent) Ben Hankinson and Brayden to get this behind us is very comforting knowing we're only one game into the season and we don't have to have this on our radar moving forward," Blues general manager Doug Armstrong said.
Selected by the Los Angeles Kings with the No. 5 pick in the 2009 NHL Draft, Schenn has 372 points (154 goals, 218 assists) in 588 NHL games with the Kings, Flyers and Blues. He has 26 points (eight goals, 18 assists) in 50 playoff games.
"He played well for us and was a big part of us winning," Blues coach Craig Berube said. "It's a good signing, good for everybody. ... He was part of a winning team here. These guys, they want to be here. It's a big thing and means a lot, which is important for the organization, and it's great that players want to be here."
Blues forward Jaden Schwartz said, "He's very well-liked in our locker room and an important piece on the ice. He's a guy that plays hard, he's a guy that plays on both ends of the ice, he digs in on draws, he can play a physical game, he can make plays offensively. Obviously everyone's really excited to see that he's locked up here for eight years."
NHL.com independent correspondent Louie Korac contributed to this report
