Thomas has one more season on his current deal, after which he would have been a restricted free agent. He will make $2.8 million against the salary cap next season before his new contract kicks in for the 2023-24 season with an average annual value of $8.125 million. The new deal is set to keep Thomas in St. Louis through the 2030-31 season.
The Blues also announced a one-year, $1.25 million contract with goaltender Thomas Greiss.
The cap hit on Thomas' new deal would make him the Blues' highest-paid player in 2023-24 -- although teammate Jordan Kyrou is also in need of a new contract after next season.
"St. Louis is where I want to be," Thomas told the team's website. "From the moment I got here, it's been a second home to me, and every year that's gone by, I've loved playing for the Blues more and more. Since I've been here, I've felt like we've had a chance at winning the Stanley Cup every season and I truly believe there will be many more opportunities to win it. That's why I couldn't be happier to be part of it for the next nine seasons."
Thomas, 23, had a breakout season in 2021-22 with 77 points in 72 games, including 57 assists. He has spent all four years of his NHL career with the Blues, totaling 42 goals and 122 assists.
A well-traveled, 2009 first-round draft pick, Leddy finished last season with the Blues, finding a fit on a club that advanced to the second round of the playoffs.
The 6-foot, 205-pound veteran is a good puck-moving defender with decent speed who excels in a third-pairing role in more protected minutes. Leddy was particularly strong in the postseason, though, with one goal and five points in nine games.
Leddy began last season with the Detroit Red Wings following an offseason trade from the New York Islanders. He has played for four teams in his career after breaking in with the Chicago Blackhawks in 2010 and is two goals shy of 70 for his career.
Leddy's deal suggests that St. Louis -- which had limited cap space to begin with -- might not have the funds to re-sign free agent forward David Perron. General manager Doug Armstrong has been shopping defenseman Torey Krug, a move that would generate some cap flexibility.
Greiss, 36, spent the past two seasons with the Red Wings. He went 10-15-1 in 31 games last season with a 3.66 goals-against average and .891 save percentage.
ESPN's Kristen Shilton contributed to this report.