Jose Mourinho's side confirmed the 30-year-old's new terms on Sunday, his previous deal having only had a year left to run.
Romero joined United on a free transfer in 2015 and has proven to be a reliable understudy to first-choice goalkeeper David de Gea.
The Argentina international started 12 matches in United's victorious UEFA Europa League campaign last season, including the final against Ajax, and made a total of 18 appearances across all competitions.
"Sergio is a great goalkeeper and professional," Mourinho told United's website.
"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 is amazing in such an important role."
Romero himself was thrilled to extend his stay with United for the long-term, describing them as the world's biggest club.
"I am delighted to have signed a new contract," he said. "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."
He has provided stability for United given De Gea has regularly been linked with a big-money move to Real Madrid and almost left for Spain in the same transfer window in which Romero arrived.
Despite being number two at club level, Romero has retained his status as Argentina's first-choice shot stopper, starting all 14 of the country's FIFA World Cup qualifiers for next year's finals in Russia.
If De Gea stays, United are well set between the sticks, with Portugal Under-21 goalkeeper Joel Pereira, who has been regularly praised by Mourinho, himself signing a new four-year deal last week.