The 23-year-old Valencia, who has played 34 times for his country, joined United as a replacement for Cristiano Ronaldo who has agreed to move to Real Madrid for a world record transfer fee.
"Playing in front of 76,000 fans alongside players like Wayne Rooney, Rio Ferdinand and Ryan Giggs will be an amazing experience. I can't wait to get started," Valencia said on United's website.
Valencia became United manager Alex Ferguson's first signing since the end of last season, reportedly costing around 17 million pounds.
"Antonio is a player we have admired for some time now. I am sure his pace and ability will make a significant contribution to the team," Ferguson said.
The Ecuadorian notched up 89 appearances, scoring seven times, after joining Wigan from Spanish club Villarreal in 2006.
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