Buys Ownership In Esports Group

Toronto Maple Leafs superstar Mitch Marner is putting his fat, new contract to good use ... by investing in a major Esports company!!

TMZ Sports has learned ... 23-year-old Marner -- who just inked a 6-year, $65 million deal -- is following The Weeknd's lead in becoming a part-owner of OverActive Media, a company that has pro Overwatch and Call of Duty teams based in the 6.

The Leafs winger is an avid gamer when he's not on the ice ... making it a business move he's really passionate about.

"Over the past few months, I’ve live-streamed with several Toronto Ultra players and built some great friendships with the team at OverActive," Marner says. "As a gamer, my investment is an opportunity to…