The 23-year-old Dutch defender has joined the Red Devils from Eredivisie outfit Vitesse Arnhem and becomes Sir Alex Ferguson's fourth signing of the summer.
The Dutchman becomes Sir Alex Ferguson's fourth signing of the summer, following the acquisitions of Shinji Kagawa, Nick Powell and Robin van Persie.
Buttner, who made 119 appearances for the Eredivisie club after making his first-team breakthrough in 2008, had been previously linked with a move to Southampton.
Buttner has been capped by the Netherlands up to the Under-20 age group and was part of Bert van Marwijk's preliminary squad ahead of Euro 2012.
Following confirmation of the deal, Red Devils boss Ferguson told the club's official website: "Alexander is one of the best young left backs in Europe and we're delighted to sign him.
"He's someone we've been watching for a while now. He gives us some really exciting options in that position."
The former Ajax youth product also expressed his delight at making the move to Old Trafford.
"Joining Manchester United is one of the best days of my life," he said. "It's a team I have admired for many years now and I can't wait to join my teammates and play my part in helping the club be successful."
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