Defender Buttner seals Man United move
The 23-year-old left-back becomes United's second recent Dutch acquistion following Robin van Persie's move from Arsenal last week.
Manager Sir Alex Ferguson told the club's 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 team-mates and play my part in helping the club be successful."
The defender was a member of the preliminary Dutch squad for Euro 2012 but did not make the final cut.