The deal for the 23-year-old Netherlands international is worth 39 million euros ($46 million), plus an additional 5 million euros ($6 million) in add-ons, Ajax said.
Van De Beek, who played an important role in Ajaxs run to last years Champions League semifinals, signed a five-year deal with the option of a further season and could form an exciting-looking midfield combination with Paul Pogba and Bruno Fernandes.
"I cannot begin to explain how incredible an opportunity it is to join a club with such an amazing history," Van De Beek said. "Everyone has told me how amazing the Old Trafford atmosphere is and I am looking forward to experiencing that, once it is safe for the fans to return."
Van De Beek came through the ranks at Ajax and has gone on to establish himself in the Dutch national team.
Van de Beek asked for the no.34 jersey as he wants to play tribute to former Ajax team-mate Abdelhak Nouri, who collapsed and suffered a cardiac attack during a pre-season game in 2017, leaaving him with serious and permanent brain damage and cutting hs career short.
Van de Beek and Nouri came up through the youth ranks together at the Ajax.
"This is a special thing for me because my good friend Abdelhak Nouri… maybe you know the story. He had a heart attack and he’s a good friend of mine, and I’m really close with his family and his brother is one of my best friends, and I talk a lot with them. So I decided to take his old number on my shirt. I want to get good memories with this number," Van de Beek said.
Many former Ajax players have also asked for Nouri's shirt number after transfer to other clubs like Justin Kluivert at Roma, Joel Veltman at Brighton and Amin Younes at Napoli.
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Our new recruit on joining United, his style of play and honouring his teammate and close friend #MUFC@Donny_Beek6
He will head to Manchester after the Netherlands’ upcoming Nations League double-header with Poland and Italy.
Donny has all of the technical attributes needed to perform in this team and has the personality required to succeed at Manchester United,” United manager Ole Gunnar Solskjaer said. His ability to see space, time his movements and read the game will really complement the qualities that we have in midfield and his arrival really strengthens the depth of talent we have in that position.
United has been heavily linked this offseason with England winger Jadon Sancho, whose club Borussia Dortmund says he will not be leaving.
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