"This club has big ambitions. I too have big ambitions," he said. "Together I'm sure we will make history."
He added: "It was always a wish for me to work in the Premier League.
"To work as a manager for Manchester United, the biggest club in the world, makes me very proud.
"I have managed in games at Old Trafford before and know what an incredible arena Old Trafford is and how passionate and knowledgeable the fans are."
United executive vice-chairman Ed Woodward is convinced he has appointed the right man this time.
"His track record of success in winning leagues and cups across Europe throughout his career makes him the perfect choice," said Woodward.
"In Louis van Gaal, we have secured the services of one of the outstanding managers in the game today.
"Old Trafford provides him with a fitting stage on which to write new chapters in the Manchester United story."
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