"It's the hardest hockey decision I've ever had to make, but I truly believe with the roster they have, with the guys they picked up this offseason, I think this team is ready to win now. I'm ready to win," Thornton said, according to TSN's Kristen Shilton.
"I need to win a Stanley Cup, and I think this is a great team that can do that," he added.
The 41-year-old has been chasing the Cup throughout his 22 NHL seasons. He lost in the finals once and reached the conference finals on three other occasions. He's amassed 133 points in 179 postseason games.
Thornton is particularly impressed by Leafs goaltender Frederik Andersen.
"I love their goalie. I like Freddie a lot," he said. "I think that was a big part of my decision, I think he's a great goalie."
Despite the star power currently on the roster, the Maple Leafs haven't reached the second round of the postseason since 2004.