The 27-year-old joins the Premier League outfit as their third signing of the summer
Borussia Dortmund have confirmed Henrikh Mkhitaryan's move to Manchester United on their official website.
The Armenian international will join Jose Mourinho at United as their third signing of the summer after Eric Bailly and Zlatan Ibrahimovic confirmed their transfers in recent weeks.
The attacking midfielder would have been out of contract next summer and Dortmund said they were made an offer by United that contained excellent value that the club could not turn down for fear of the player moving on a free transfer next season.
Last year's Bundesliga runners up are in a rebuilding phase having lost Ilkay Gundogan, Mats Hummels and now the 27-year-old Mkhitaryan who landed 23 goals and 32 assists last season for Thomas Tuchel's side.
The German club wished their former player well as he competes in the Premier League and Europa League next season.