Manchester United made the long-expected appointment of former Chelsea, Real Madrid, Inter, and Porto manager official on Friday, announcing the move via their Twitter account.
"José is quite simply the best manager in the game today. He has won trophies and inspired players in countries across Europe and, of course, he knows the Premier League very well, having won three titles here," the club's executive vice chairman, Ed Woodward, said.
"I’d like to take this opportunity to welcome him to Manchester United. His track record of success is ideal to take the club forward."
Mourinho said it was a "special honour" to be named manager of Manchester United.
"It is a club known and admired throughout the world. There is a mystique and a romance about it which no other club can match," he said.
"I have always felt an affinity with Old Trafford; it has hosted some important memories for me in my career and I have always enjoyed a rapport with the United fans. I’m looking forward to being their manager and enjoying their magnificent support in the coming years."
Mourinho replaces the 64-year-old Dutchman Louis van Gaal, whose sacking was announced on Monday, just two days after United beat Crystal Palace 2-1 to win the FA Cup.