The agreement is worth $167 million, MLB.com reported Friday.
Posey, 26, was the National League Rookie of the Year in 2010, when he hit .305, stroked 18 home runs, drove in 67 runs and helped the Giants win the World Series.
He missed most of 2011 because of a leg fracture and torn ankle ligaments, sustained in a home-plate collision, but he bounced back in 2012 -- with a league-leading .336 batting average, to go with 24 home runs and 103 RBI -- and was voted MVP in a landslide.
The new deal is said to be the biggest contract in franchise history, passing the deal pitcher Matt Cain signed worth $127.5 million over six years. It's also seen as the biggest contract for a catcher.
In four years in the major leagues -- all with the Giants -- Posey has hit .314 with 46 home runs and 191 RBI.