Webster will receive $20 million in guaranteed money under the deal that runs through 2013. The fourth-year player would have been a free agent at the end of the season.
"Corey has been a outstanding person and player in his short career for the Giants," general manager Jerry Reese said. "We are very happy to get this deal done."
Webster has started every game at right cornerback this season. He has three interceptions, tied with fellow cornerback Aaron Ross for the team lead. He also has two forced fumbles.
"He really came into his own the latter part of last year, and that has carried over into the 2008 season, where he has shown that he is one of the top corners in the league," Reese said Sunday evening.
Webster was the Giants' second-round draft choice in 2005, the 43rd overall selection. After being benched early in the year, he moved into the starting lineup for the playoffs when Sam Madison suffered a sports hernia.
Webster held Tampa Bay's Joey Galloway to one catch in the NFC wild-card game. A week later, he had five tackles, broke up a pass and helped limit Dallas to 182 yards passing in the divisional playoff.