Ajax had reached an agreement with United earlier this month with the 24-year-old moving for an initial fee of 57.37 million euros and a further 10 million euros of potential add-ons included.

United said they also have the option to extend his contract for a further year. Primarily a centre back, Martinez can also slot into midfield and he is reunited with manager Erik ten Hag (56), who coached him for three seasons at Ajax.

"It's an honour to join this great football club. I've worked so hard to get to this moment and now that I'm here, I'm going to push myself even further," said Martinez, who was Ajax's player of the year last season.

"I've been lucky enough to be part of successful teams in my career and that's what I want to continue at Manchester United.

"There will be a lot of work to get to that moment, but I firmly believe that under this manager and coaches, and together with my new teammates, we can do it."

United finished sixth last season, conceding 57 goals - the worst defensive record among the top six teams.

They begin the new season with a home game against Brighton & Hove Albion on August 7, something you can follow on Flashscore>>>