In exchange, the Coyotes will receive prospect Maxim Letunov, who was a second-round pick by the Blues in the 2014 draft.
Michalek has not played since Feb. 14 due to a concussion. In 53 games this season, the 32-year-old has two goals and six assists in 21:05 average time on ice.
Michalek has played for the Coyotes for eight years and has also spent time with the Minnesota Wild and Pittsburgh Penguins, totaling 30 goals and 103 assists in his 11-year career. He is in the final season of a five-year deal and has a $4 million salary in 2014-15.
Letunov was in his second season with the Youngstown Phantoms of the USHL prior to the trade. In 47 games this season, he has 22 goals and 31 assists.
The NHL trade deadline is 3 p.m. ET on Monday.
- Molly Geary