The 27-year-old has signed a four-year deal and is Jose Mourinho‘s third summer signing.
United had an initial offer of £18m rejected, but went back in after Mourinho made it clear to executive vice chairman Ed Woodward how desperate he was to sign him.
Woodward jetted to Monaco on Tuesday to meet Mkhitaryan’s agent Mino Raiola and put the finishing touches to the deal.
Mkhitaryan, who signed a contract worth £100,000-a-week, said: “I am very proud to join Manchester United, this move is a dream come true for me.
“I am excited to play for a club with such an illustrious history and hope to be part of it for a long time.