Louis, 62, has managed at the top level of European football for over 20 years and in that time has won domestic titles and domestic cups in three countries, as well as the UEFA Champions League, the UEFA Cup, an Intercontinental Cup, two UEFA Super Cups and domestic Super Cups in Holland and Germany.
He will take up his post after the FIFA World Cup, where he will manage the Dutch national team.
Ryan Giggs, English football's most decorated footballer and the United player with the most appearances in the club's history, has been appointed as assistant manager. Ryan took charge of the first team for the last four matches of the 2013/14 season.
Announcing the appointments, Ed Woodward said: "In Louis van Gaal, we have secured the services of one of the outstanding managers in the game today. He has achieved many things in his career to date and Old Trafford provides him with a fitting stage on which to write new chapters in the Manchester United story.