"A Christmas morning we'll never forget," Ryan tweeted. "The Giants and I just agreed to a new three-year deal. I'm here to stay. Merry Christmas."
Ryan's deal is for $31 million, with $20 million guaranteed.
The eight-year veteran signed a one-year contract with the Giants last off-season. He has 83 total tackles, nine passes defensed, three forced fumbles, a sack, an interception and a fumble recovery in 14 games this season.
The two-time Super Bowl champion entered the league as a third-round pick by the New England Patriots in the 2013 NFL Draft.
Ryan, 29, spent his first four seasons in New England before he spent the 2017 through 2019 seasons with the Tennessee Titans.
The Giants next face the Baltimore Ravens at 1 p.m. EST on Sunday at M&T Bank Stadium in Baltimore.