The 28-year-old, who played under Van Gaal at Dutch side AZ Alkmaar, joins United as a free operators subsequent to leaving Sampdoria toward the end of his agreement.
"To play for the greatest club on the planet is a blessing from heaven for me," he told United's site.
"Louis van Gaal is a phenomenal supervisor and I can't hold up to begin on this new and energizing test in my vocation."
Romero, who has a possibility for an extra year on his agreement, has joined United on their pre-season voyage through the United States, where they beat Barcelona 3-1 on Saturday.
"Sergio is an exceptionally gifted goalkeeper. He was a youthful manager amid my time at AZ Alkmaar and I am enchanted he is joining Manchester United," Van Gaal said.
Romero and his new administrator likewise happened to meet finally year's Reality Glass, where Argentina disposed of the Van Gaal-guided Netherlands in a semi-last punishment shootout.
"He had an awesome World Container the previous summer in Brazil, despite the fact that that is something I have set to the back of my brain. He will be an incredible expansion to the group and I am anticipating working with him by and by," Van Gaal included.
Romero's landing is required to proclaim the flight of United's reinforcement Victor Valdes, who was let alone for the visit gathering and advised to leave the club because of a column with the chief.
Van Gaal said Valdes had declined to play for United's store group, a case questioned by the Spaniard.
Theory additionally proceeds over the eventual fate of first-decision United attendant David De Gea, who has been connected with a move to Genuine Madrid, with the La Liga group's shield Sergio Ramos making a beeline for the English side as a major aspect of a swap bargain.
"I know him (Romero) extremely well and I think in our circumstance, if De Gea is going or not going, it is dependably a danger and we need to fare thee well, verify that we are readied and that is the reason I think it is great that Sergio Romero is currently a Manchester United player," Van Gaal included.
United top their United States visit on Wednesday against Paris St-Germain, who are purportedly enthusiastic about marking the Chief Association club's unsettled midfielder Holy messenger Di Maria.
The Argentine did not go with United on their visit, with Van Gaal conceding he "doesn't know why".
Romero turns into United's fifth close season marking after the landings of Italy shield Matteo Darmian, Dutch forward Memphis Depay, French midfielder Morgan Schneiderlin and German captain Bastian Schweinsteiger.