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 join one of the best global esports organizations and to help bridge the gap between hockey fans and esports."

"I’m looking forward to working with the players and the front office to introduce the Toronto Ultra and Toronto Defiant to a young audience of traditional sport fans."

OverActive CEO Chris Overholt added ... "With roots in the city of Toronto, our entire organization is thrilled to welcome a hometown hero and hockey superstar like Mitch Marner to our investor family,"

"Mitch's authenticity within the Call of Duty scene and his interest in growing the esports industry will be such great additions to our fan base and to each of our franchises."