Romero was a free agent after leaving Sampdoria and will replace back-up keeper, Victor Valdes, who has been told he can leave the club.
The 28-year-old, who sealed a three-year deal, is United’s fifth signing of the summer, having signed full-back, Matteo Darmian, midfielders Bastian Schweinsteiger, Morgan Schneiderlin and forward, Memphis Depay.
Romero has 62 caps with the Argentine national team and played in the 2014 World Cup final, he also won the 2008-2009 Eredivisie title under Van Gaal at Az Alkmaar.
Addressing reporters about the new goalkeeper, Manchester Utd team boss, Louis van Gaal, said Romero did very well in the World Cup.
He said: “Sergio Romero is an international goalkeeper from the Argentina team. He was also a former player of mine. I was trainer-coach, manager of AZ Alkmaar in the Netherlands, I picked him up when he was 19 and he was a great talent. We became champions with him, with a minor club, and in the Netherlands it was a fantastic result of course.
“I know him very well and I think in our situation, if De Gea is going or not going, it is always a threat and we have to take care, make sure that we are prepared and that is why I think it is very good that Sergio Romero is now a Manchester United player”.