McMichael, 25, was originally dealt to the Blues in a trade with the Washington Capitals earlier this offseason in exchange for veteran Jordan Kyrou.
St. Louis also acquired the 16th overall pick in this year's draft along with Milton Gastrin.
McMichael spent six years with the Caps after the team selected him with the 25th overall pick in the 2019 NHL Draft. The young center has played in at least 78 games in each of the past three seasons, scoring 18-26-14 goals, respectively.
He's coming off a season where he set a career-high with 32 assists.
The Ontario native will now become a crucial piece in the Blues' rebuild as they look to leave behind a season in which they finished 37-33-12, missing the playoffs for the third time in the past four seasons.