The midfielder underwent medical tests in Holland earlier this week and after they showed no issues.
He has now penned a five-year contract, with the option for an additional 12 months at Old Trafford.
The Red Devils will pay Ajax £34.5m upfront with a potential further £4.5m in add-ons.
Van De Beek delighted for the move
Donny Van De Beek, who will wear the No.34 shirt at Old Trafford said that it is a great opportunity to join a club like Manchester United.
“I cannot begin to explain how incredible an opportunity it is to join a club with such an amazing history,” Van De Beek said before adding.
“I would like to thank everyone at Ajax, I grew up there and I will always have a special bond with the club.”
“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,” he added.
The box-to-box style player scored ten goals from midfield last term and is seen as the perfect foil for Paul Pogba and Fernandes by Solskjaer.
He is now eager to replicate his Ajax form to the Premier League giants.
“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.”
“This team has some of the best midfielders in the world and I know that I can learn from them and also bring my own strengths to the group.”
“Having spoken to the manager about his vision for this team, the direction that the club is going in is hugely exciting and I cannot wait to be a part of that,” he noted.
Manchester United targets titles ahead of the 2020/2021 season after finishing last season trophyless.