The 39-year-old Portuguese, who will move to Old Trafford from Lisbon club Sporting on 11 November, has signed a contract until June 2027.
Former United striker Ruud van Nistelrooy, who took charge on an interim basis after Erik ten Hag was sacked on Monday, will stay on for the club’s next three games
Amorim is the sixth permanent manager United have appointed since Sir Alex Ferguson’s 26-year reign ended with his retirement in 2013.
In a statement, the club said that “Ruben is one of the most exciting and highly rated young coaches in European football”.
Earlier in the week, Sporting said that United had agreed to pay 10m euros (£8.3m) to trigger a release clause in Amorim’s contract.
Amorim’s first fixture at Manchester United will be on 24 November against newly promoted Ipswich, which comes after the international break.
His first home game will be against Norwegian side Bodo/Glimt in the Europa League on 28 November, with a Premier League game against Everton the following weekend.
Manchester United will announce who will be on Amorim’s coaching staff at a later date.
After losing against rivals Porto in the Portuguese Super Cup on 3 August, Amorim has guided Sporting through a 14-game unbeaten run.
Amorim is due to be in charge tonight when Sporting host Estrela in a league game.
Sporting will host Manchester City in the Champions League on Tuesday and play Amorim’s former club Braga in the league on 10 November, before European top-flight football pauses for Nations League games.
Editor Paul Akhagbemhe
