The forward, who turned 40 on July 2, had 51 points (16 goals, 35 assists) in 73 regular-season games with the Sharks last season and 10 points (four goals, six assists) in 19 Stanley Cup Playoff games.
"It wasn't too long after the season ended, I started getting back into the gym and feeling good, so it was definitely before July," Thornton said of when he knew he wanted to play another season. "I'm not sure exactly, but it's been a while."
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Thornton was mostly healthy last season after having major knee surgery prior to each of the past two seasons. He said not having to spend this offseason rehabilitating an injury was a contributing factor in his decision to return.
"These last few summers I had to go to rehab every day and I really couldn't focus in on what I needed to do, and really I wasn't skating until later on because my knee just wasn't ready," he said. "This summer I was skating for a month and a half now, getting stronger. My body feels really, really good."
Acquired in a trade with the Boston Bruins on Nov. 30, 2005, Thornton is the Sharks leader in assists (780), is second in games (1,034) and points (1,024), and is third in goals (244).
"I've been in this business a long time and I've never met a guy loves the game like him, loves the team, makes everybody around him better," Sharks general manager Doug Wilson said. "It's a privilege for all of us to be around him because he's contagious in how he's impacted this franchise not only on and off the ice, but his teammates and all the young guys have had the ability to learn and watch and see what it takes to truly be great.
"It's a great way to start the season, not only with 'Jumbo' signing but with having everybody under contract and ready to go."
The Sharks re-signed several key players this offseason, including defenseman Erik Karlsson to an eight-year contract on June 17, and restricted free agent forwards Timo Meier (four-year contract) on July 1 and Kevin Labanc (one-year contract) on July 8.
"I told [Doug], 'You figure out your stuff and then come find me,' so I was in no hurry. ... We talked all summer. He knew I was going to come back, and I knew I was going to come back," Thornton said.