The Argentina international, 30, has been on the Red Devils" books for two years since moving to England from Serie A side Sampdoria.
And he"s since racked up 28 first-team appearances, including 12 starts as Jose Mourinho"s men won the Europa League last season.
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Romero has been linked with a return to his homeland this year, with Boca Juniors said to be keen on securing his services.
However, United have now announced they have tied him down for another four years, with the option to extend for a further 12 months.
Opening up on the deal, Romero said: "I am delighted to have signed a new contract. Who wouldn"t want to be at the biggest club in the world?
"We have great goalkeepers here and we are always learning from each other.
"Of course, it was a very proud moment for me and my family to be part of the Europa League final, and I am looking forward to helping the team build on last season"s success in 2017/18."
Mourinho offered his opinion on Romero in the wake of the news, broken amid United"s pre-season tour of the USA.
He said: "Sergio is a great goalkeeper and professional. He had a good season and outstanding performances, especially in our Europa League success.
"Sergio deserves his contract, and for Manchester United to have Sergio and David [De Gea] is amazing in such an important role."
