In exchange for Schenn, a three-time 20-goal scorer, the Flyers got the Blues' first-round pick this year, a conditional first-round pick in next year's draft and third-year forward Jori Lehtera.
The Flyers used the Blues' pick, 27th overall, to select center Morgan Frost.
The Blues weren't done dealing. They traded back into the first round by acquiring the No. 31 pick from the Stanley Cup champion Pittsburgh Penguins; that pick was used to select center Klim Kostin. St. Louis also acquired rookie forward Oskar Sundqvist.
The Penguins added toughness by acquiring seven-year veteran forward Ryan Reaves and the Blues' second-round pick, 51st overall.
In Schenn, the Blues add a playmaker who had 25 goals and 30 assists for 55 points in 79 games last season. Overall, he has 109 goals and 246 points in 424 games over eight NHL seasons.
Schenn carries a hefty contract, which represents a $5.125 million cap hit in each of the next three seasons.
Lehtera was selected in the third round of the 2008 draft. He had seven goals and 22 points in 64 games last year.