The 23-year-old Reimer went 20-10-5 in 37 games after getting called up from the AHL. He scored the big raise after finishing off a rookie contract that paid him about $600,000 at the NHL level, and is expected to be the team's No. 1 goalie next season.
Reimer would have become a restricted free agent July 1.
"He earned the opportunity to play at the NHL level this past season and he made the most of it," Maple Leafs general manager Brian Burke said in a statement. "He really grabbed the net and never looked back. We are confident that his talent and strong work habits will be contributing factors to our team's success next season."
Burke still has a decision to make on veteran goalie Jean-Sebastien Giguere, who had offseason sports hernia surgery and is due to become an unrestricted free agent July 1.
Jonas Gustavsson is also under contract for next season.