The deal, announced Tuesday, is for $4.5 million per season and extends through the 2023-24 season.
Zaitsev, 25, had a solid rookie season in 2016-17, registering 36 points (four goals, 32 assists) in 82 games and leading Toronto players with an average of just over 22 minutes of ice time per game.
Prior to signing with Toronto, Zaitsev played in 317 regular-season games in the Kontinental Hockey League, registering 95 points (32 goals, 63 assists).
The Leafs also announced a one-year deal with forward Ben Smith worth $650,000. Smith, 28, had four points (two goals, two assists) in 36 games with the Maple Leafs last season after being claimed off waivers from Colorado.