Ajax confirmed the deal is worth €39 million, plus a potential €5 million in add-ons.
Van de Beek put pen to paper on a five-year contract with the Red Devils after completing a medical in the Netherlands.
He'll now join the Dutch national team ahead of Nations League matches against 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," manager Ole Gunnar Solskjaer said in a statement. "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."
The 23-year-old - who played a key role in Ajax's run to the Champions League semifinals in 2018-19 - drew interest from Real Madrid before receiving an offer from United.
Barcelona were reportedly interested in signing Van de Beek, a personal favorite of newly appointed Dutch manager Ronald Koeman, but were unable to finance a move.
A two-way midfielder who can sweep up and join the attack, Van de Beek made his senior debut as an 18-year-old in the Europa League against Celtic. He earned his first start in the Eredivisie three days later in a win over PEC Zwolle.
Overall, Van de Beek recorded 41 goals and 34 assists in 175 appearances with Ajax.
"I would like to thank everyone at Ajax. I grew up there and I will always have a special bond with the club," Van De Beek said. "I am now ready to take the next step in my career and perform at the highest level, and there is no higher standard than Manchester United."
Solskjaer now has a ton of creativity in his squad. With Bruno Fernandes, Paul Pogba, and Van de Beek combining in midfield, the Red Devils have an envious rotation of players to feed the likes of Marcus Rashford, Mason Greenwood, and Anthony Martial.
A bid for Borussia Dortmund winger Jadon Sancho, however, has yet to materialize.