Carlo, who has one season remaining on a six-year, $24.6 million contract ($4.1 million average annual value) he signed with the Boston Bruins on July 14, 2021, can become an unrestricted free agent after next season.
Carlo had seven assists in 55 games for the Maple Leafs this season. He was acquired by Toronto in a trade with Boston on March 7, 2025, for forward Fraser Minten and two draft picks.
"We’re excited that he’s got size and length, his ability to kill plays, his experience, I think," Blues general manager Doug Armstrong said. "It allows us to have four experienced players right now in (Philip) Broberg, (Colton) Parayko, (Cam) Fowler and Carlo now, and then we have those younger guys that are going to push and prod and try to work their way in there (Logan Mailloux, Theo Lindstein, Adam Jiricek).
"The way we were set before, we were going to rely on three young players to take a big role and things that we tried to accomplish the last few days of getting stronger up front and having strong goaltending, we think we’re going to be more competitive than we were last year, and adding a defenseman there that can make us stronger and also provide us the ability to let the younger players to come at a more natural pace and also provide us depth for injuries was important for us."
Selected by Boston in the second round (No. 37) of the 2015 NHL Draft, Carlo has 119 points (29 goals, 90 assists) in 692 regular-season games for the Bruins and Maple Leafs and 14 points (five goals, nine assists) in 85 Stanley Cup Playoff games.
Carlo will be reunited with St. Louis coach Jim Montgomery, who he played under with the Bruins from 2022-24.
"'Monty' speaks very highly of his time with him, even higher of his character of how he fits into a team game," Armstrong said. "That’s an added benefit of having that extra knowledge."
The move is the latest by Toronto, which acquired goalie Samuel Ersson (later traded to the Ottawa Senators) and defenseman Emil Andrae in a trade with the Philadelphia Flyers on June 16 for goalie Joseph Woll and Simon Benoit. The next day, the Maple Leafs hired Jim Hiller as coach, replacing Craig Berube.
Two days later, the Maple Leafs acquired defenseman Darren Raddysh in a trade with the Tampa Bay Lightning for Toronto's fifth-round selection in the 2026 draft. Raddysh then signed an eight-year contract with Toronto.
The Maple Leafs also selected forward Gavin McKenna with the No. 1 pick on Friday.
The Blues acquired forward Mason McTavish from the Anaheim Ducks on Friday for two first-round picks in the 2026 draft. They also traded forward Jordan Kyrou to the Washington Capitals on Tuesday for forwards Connor McMichael and Milton Gastrin, and a first-round pick this year, which was later traded to the Ducks.
NHL.com independent correspondent Lou Korac contributed to this report
