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 ($58.21 million) and a further 10 million euros of potential add-ons included. Read full story
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, who coached him for three seasons at Ajax.
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 Aug. 7.