The veteran Welshman had already indicated his wish to continue his career into a remarkable 21st season at Old Trafford and United have confirmed the deal has been signed.
Giggs has made 24 appearances for the Red Devils this season and his glittering career includes 11 Premier League title medals.
LUCKY ENOUGH
"All I've ever wanted to do is play for United and I've been lucky enough to do that for 20 years," said Giggs.
"It is great to know I'm still contributing to the team's success and I feel I've got a lot to offer on and off the pitch.
"This is an exciting time to be involved with so many good young players coming through."
Giggs has played more than 850 times for the Red Devils and recently featured in his 600th Premier League match. He also holds the distinction of scoring in every Premier League campaign.
"You run out of words to describe Ryan Giggs," said manager Sir Alex Ferguson. "He is a marvellous player and a wonderful man.
"To have the desire and the ability to play at the top level in such a physically demanding position at his age requires a special person.
"He is still turning in man-of-the-match performances and his experience is so vital for the younger players in the squad."