Kostin, 18, was drafted 31st overall by the Blues at the 2017 NHL Entry Draft. The 6'3, 196-pound forward spent 2016-17 playing professionally in Russia, including an eight-game stint with the Kontinental Hockey League's (KHL) Dynamo Moskva. In 2015-16, Kostin tallied 21 points (eight goals, 13 assists) in 30 games with HK MVD Balashikha.
Internationally, the Penza, Russia, native captained Team Russia at both the 2015 World U-17 Hockey Challenge and the 2016 U-18 World Junior Championships.
At the 2015 Hockey Challenge, Kostin posted five points (two goals, three assists) in six tournament games, leading Russia to the silver medal.
