The 28-year-old has signed a three-year deal with the Red Devils on a free transfer after leaving Italian club Sampdoria earlier this summer.
“I am absolutely delighted to join Manchester United," he told the club's official website.
"To play for the biggest club in the world is a dream come true for me.
"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 been capped 64 times by Argentina including the losing World Cup final to Germany in Brazil last year and the unsuccessful Copa America final against Chile earlier this summer.
BREAKING: We're pleased to announce Argentina goalkeeper Sergio Romero has signed on a three-year deal #WelcomeRomeropic.twitter.com/rnFiZXvoxH
— Manchester United (@ManUtd) July 27, 2015