Coughlin, 67, told reporters at the NFL scouting combine in Indianapolis about the deal, which will keep him in New York through the 2015 season.
The Giants lost their first six games last season before finishing with a 7-9 record.
"I really haven't paid a lot of attention to it; it's not a big deal," Coughlin told NFL.com. "A year has been added to the contract."
In 10 seasons with the Giants, Coughlin has a 90-70 record with three division titles and two Super Bowl championships.