United agreed a deal to sign Diallo from Atalanta during last summer’s transfer window, with the move subject to a medical, work permit and personal terms.
Amad Diallo is officially a Manchester United player after the club announced his arrival on Thursday.
All three hurdles were overcome and Diallo will link up with his new United teammates in Manchester when his visa is processed.
United manager Ole Gunnar Solskjaer said: “As a club, we have followed Amad for a number of years and having watched him myself, I believe he is one of the most exciting young prospects in the game.”
Diallo has signed a contract to the end of June 2025, with the option of an extra year.
United bought fellow teenage winger Facundo Pellistri from Penarol on the same day as Diallo but the Uruguayan moved to England straightaway and has played eight times for the Under-23s and Under-21s.
United are preparing for an FA Cup clash with Watford this weekend.