ESPN's Pierre LeBrun first reported the trade.
In return for Jokinen, the Maple Leafs received forward Joakim Lindstrom and a conditional sixth-round draft pick.
It is the second trade in less than a month for Jokinen, who was dealt to Toronto by the Nashville Predators on Feb. 15. The Blues will be the 10th NHL team that the 36-year-old has played for.
Jokinen signed a one-year, $2.5 million deal with Nashville last July. In 54 games this season, he has three goals and four assists with a plus-three rating in an average of 13:36 time on ice.
The NHL trade deadline was 3 p.m. ET on Monday.
- Molly Geary