Valencia, who is currently playing for Ecuador at the World Cup finals in Brazil, joined United in August 2009 and has made 191 appearances for the club.
Assistant manager Ryan Giggs told the club's official website: "I'm really pleased Antonio has signed a new contract. His contribution to the team since he joined the club in 2009 has been fantastic.
"Antonio is a very powerful player with great speed and versatility. He is very professional in his approach to training and I am delighted he has committed his future to the club."
Valencia, who has won two Premier League titles during his time at Old Trafford, said: "My time at Manchester United has been like a dream come true. To be the first Ecuadorian player to win the English Premier League was a very proud moment for me and for my country.
"I am so happy to have signed a new deal and am looking forward to working with Louis van Gaal. There is great team spirit within the club and we are all determined to get back to winning ways."
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