The deal was announced on the eve of training camp.
Schenn, 21, played all 82 games last season, collecting five goals and 17 assists.
The Saskatoon native is entering his fourth season with the Maple Leafs.
"I've been given nothing but a great opportunity here," said Schenn. "I'm looking forward to the next five years here in Toronto."
Schenn, who was Toronto first-round pick, fifth overall, in the 2008 NHL entry draft, finished a three-year, $2.625 million contract at the end of last season.
In his three years with Toronto, he has 12 goals and 41 assists for 53 points.