Agent Ian Greengross confirmed the deal Saturday. It is worth $13 million and includes $3.3 million in guaranteed money.
Fields' contract was set to expire after the 2012 season. The extension will put him under contract through 2016.
The 2007 seventh-round pick out of Michigan State is coming off his best NFL season. He set single-season franchise records with a 48.8-yard average and a net of 41.4 yards.
The only player on the Dolphins roster with a longer tenure is long snapper John Denney, who joined Miami as a rookie free agent in 2005.