On June 23, 2017, at the 2017 NHL Entry Draft, Schenn was traded by the Flyers to the St. Louis Blues in exchange for Jori Lehtera, a 2017 first-round pick (27th overall) and a conditional 2018 first-round pick. In his first season with the Blues, Schenn broke out and put up career highs in points, assists, and goals. He recorded his third career hat trick on December 5, 2017, in a 4-3 win over the Montreal Canadiens and lead the Three Stars of the Week. The following month, Schenn was named to the 2018 National Hockey League All-Star Game after recording 42 points in 32 games. He finished the season with a career-high 70 points in 82 games.