The 28-year-old will replace Victor Valdes, who was shown the door at Old Trafford after apparently refusing to play for United’s second team.
“To play for the biggest club in the world is a dream come true for me,” said Romero in a statement on the club’s website.
“Louis van Gaal is a fantastic manager and I cannot wait to get started on this new and exciting challenge in my career.”
Romero has made 62 appearance for Argentina and was part of Argentina’s squad for the 2014 World Cup in Brazil where they finished runners-up to Germany.
He will already be familiar with the methods of Manchester United manager Van Gaal, who was coach during Romero’s time at Dutch club AZ Alkmaar.
“He was a young keeper during my time at AZ Alkmaar and I am delighted he is joining Manchester United,” Van Gaal said on the club’s website.
“He will be a great addition to the team and I am looking forward to working with him once again.”
What can United fans expect from Romero?
“He is a very dominating goalkeeper, he dominates the 60-metre area,” Van Gaal said. “His stature is also tall and he gives a lot of confidence to the players.
“Of course, he has to learn English because that is also a problem always with a foreigner. He is very good with his feet, so he fits in our profile. I have a lot of confidence that he can fulfil what we are expecting.”
By all accounts Romero is an ok goalkeeper but he is not in the Victor Valdes class. Which means that United are actually weaker at goalkeeper today than they were when the season finished. There is no indication that Romero is ready to be a No.1 in the Premier League, meaning that David de Gea will not be sold this summer.