King, who swept the board at the club's player of the year awards in April, was outstanding as the Gills won promotion back to League One at the first attempt last season.
The 26-year-old, who arrived from Barnet for £200,000 in the summer of 2007, had 12 months left on his previous deal, but manager Mark Stimson has moved to secure him until the summer of 2012.
Stimson told KentNews: "It's a big signing for us. Although he still had a year to go on his contract, to get him for that length of deal is a major plus. If the club is going to go forward we need players like Simon.
"He's been absolutely outstanding since I've been at the club and you always have a concern when someone's in the last year of their contract.
"With how he played in the play-offs live on TV you keep your fingers crossed no one comes in for him, but this puts all that to bed now."