The 23-year-old has signed a five-year contract that will keep him at Old Trafford until the summer of 2017.
United boss Sir Alex 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.”
Buttner said: “Joining Manchester United is one of the best days of my life.
“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.”
