Later, Gronkowski's agent, Drew Rosenhaus, confirmed the deal via his Twitter account.
"Congrats to Gronk on his extension with the Patriots," Rosenhaus wrote. "My thanks to Mr.Kraft & Coach Belichick for making this possible."
The Patriots rewarded Gronkowski after his emergence as one of the top young tight ends in the game.
The former University of Arizona standout had two years and $1.175 million remaining on his rookie contract.
He's now under contract through the 2019 season.
With Gronkowski locked up for nearly the remainder of the decade, the Patriots have one of the most prolific passing tandems in the league between him and star quarterback Tom Brady.
Gronkowski, 23, caught 90 passes for 1,327 yards and 17 touchdowns last season.
He set a total for most receiving yards by a tight end in a single season, breaking Kellen Winslow's 1980 record of 1,290 yards.
New Orleans Saints tight end Jimmy Graham finished with 1,310 yards.
In just two NFL seasons, Gronkowski has generated 132 career receptions for 1,873 yards and 27 touchdowns.
He had offseason ankle surgery.
He's expected to return in time for training camp.
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