Terms of the contract weren't revealed, but ESPN reported it could reach a maximum of $42.5 million and includes $22.5 million in guaranteed money.
Stewart, a first-round 2008 NFL Draft pick out of Oregon, racked up 761 yards on 142 carries and four rushing touchdowns last season, adding 47 catches for 413 yards and a score.
Stewart "is a playmaker who can make the difference in games, and we have seen the impact he and (fellow running back) DeAngelo Williams have had on our team," Panthers General Manager Marty Hurney said.
"I'm happy and blessed to have this opportunity," Stewart added.
He has amassed 3,500 rushing yards on 725 carries with 26 TDs over his four-year career.
Williams signed a five-year extension last season.