It came on the first day of training camp on Wednesday.
Financial terms were not disclosed.
""We've extended his agreement by two years, making it a seven-year agreement," said team owner Robert Kraft.
Belichick originally signed a five-year deal in 2000. He went 5-11 in his first season before rebounding to an 11-5 Super Bowl championship season in 2001.
Last season, his team finished in a 9-7 tie for first place in the AFC East. They missed the playoffs by virtue of the tie-breaking system.
"It's a real thrill for me to be a part of the Kraft Family and this organization," Belichick said. "I'm pleased that Mr. Kraft has chosen to extend my agreement here."
Belichick is 25-23 with New England, and was 36-44 in five years as head coach of the Cleveland Browns from 1991-1995. His overall head record is 61-67.