The 39-year-old Portuguese manager has agreed to a deal through June 2027 and will join Old Trafford from Lisbon club Sporting on November 11.
Ruud van Nistelrooy, a former United striker who took over as temporary manager after Erik ten Hag was fired on Monday, will remain with the team for their next three games.
Amorim is United’s sixth permanent manager since Sir Alex Ferguson retired in 2013, ending his 26-year tenure.
In a statement, the club said that “Ruben is one of the most exciting and highly rated young coaches in European football”.
Sporting reported earlier this week that United had consented to pay 10 million euros (£8.3 million) to activate a release clause in Amorim’s contract.
Following the international break, Amorim’s debut game at Manchester United is scheduled for November 24 against recently promoted Ipswich.
On November 28, he will play his first Europa League match at home against Bodo/Glimt of Norway. The following weekend, he will play Everton in the Premier League.
