"We are creating the conditions for a brilliant and successful future for Manchester United. I love my players and it is a pleasure to know that we are going to be together for at least the next three years."
United's executive vice-chairman Ed Woodward praised Mourinho for his "exceptional work rate and professionalism", as well as for embracing the club's desire to promote young players.
"He has brought an energy and a sense of purpose to everything that he does and I am sure that will continue to bring results for the fans and the club," Woodward added.
United are second in the Premier League, behind runaway leaders Manchester City, with the club still competing in the Champions League and FA Cup.