Signing the teenage seasonal winger, the Manchester United manager, Ole Gunnar Solskjaer, said that he has landed a future star.
Diallo has signed a four-and-a-half-year deal, which will make him Manchester United player till June 2025.
United first shown interest in Diallo in 2016, as the youngster has developed and broken through at Atalanta, playing five games for the first-team of the club, and scored goal.
Though, a deal was agreed in October but there was no work permit to see it through.
The transfer is said to be around £37million but the manager believes that United is getting a rare talent.
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.
“Manchester United has such a proud history of developing young players and everything is in place to enable Amad to reach his potential here.
“It will take time for him to adapt but his speed, vision and fantastic dribbling ability will stand him in good stead to make the transition. He is a player with all of the raw attributes that are needed to be an important player for Manchester United in the years to come.”
Diallo, who was excited with the move, said: “After waiting since the summer, it is finally a dream come true to complete my move to Manchester United. I am hugely ambitious and there is so much that I want to achieve in the game; when I spoke to the manager, I knew that I had joined the perfect club.
“This is a really exciting squad with so many great players, I can promise that I will work hard every day to learn from them and give everything to develop my game. The coaches have been fantastic since I signed, we have been in touch regularly and they have already given me a lot of great advice“.
The 18-year-old winger is expected to join the rest of the team as soon as he receives his visa.