The deal, which will take Giggs beyond his 40th birthday, has been agreed after an outstanding series of performances from the veteran and will stop speculation that he would retire at the end of the season.
The midfielder is already the most decorated player in British football history and will be winning his 13th league championship should United press home their 12-point advantage over Manchester City in the Barclays Premier League.
His next appearance will be his 1,000th senior appearance, which includes games for Wales and Team GB at the London Olympics.
Sir Alex Ferguson said: "What can I say about Ryan that hasn't already been said? He is a marvellous player and an exceptional human being. Ryan is an example to us all, the way in which he has, and continues to, look after himself.
"He has fantastic energy for the game and it is wonderful to see. Ryan seems to reach a new milestone every week and to think that he now has 23 unbroken years of league goals behind him is truly amazing in the modern-day game.
"His form this year shows his ability and his enjoyment of the game are as strong as ever and I am absolutely delighted that he has signed a new contract."
Giggs told Manchester United's official website: "I am feeling good, enjoying my football more than ever and, most importantly, I feel I am making a contribution to the team. This is an exciting team to be part of, with great team spirit, and we are again pushing for trophies as we head towards the business end of the season.
"I am delighted to have signed a new contract. I would like to thank Sir Alex and the coaching staff and I would also like to thank David Gill who has been a tremendous support over my many years here."
@umahahahe Man alive you're hard to impress! Of course people are making a fuss, it is exceptional. You can name the number of players who have played at the highest level, for a top club (Champions League) beyond 40 on one hand, the hand of someone the victim of a foolish lawnmower mishap. Of course Giggs can't play every game and he doesn't reach the heights he used to, but he still has moments of brilliance, for example the pass the RvP for the equaliser v WHAM in the FA Cup. The fact that the best manager in the game still thinks Giggs has something to offer as a player is testament enough to how remarkable he is. Ferguson has shown many times he's not into sentimentality, if Giggs wasn't deemed good enough, he wouldn't have been offered the contract. I can make no comment on Giggs the man, I've never met him, but as a player he has had a career matched only by Paolo Maldini in the history of football.- solskjaer99