Casemiro has had a fantastic career at Real Madrid, including five Champions League titles.
He has also made 63 international appearances for the Brazilian national team.
Now he will strengthen the crisis team Manchester United's midfield.
The English big club, which has a new coach in Erik Ten Hag, has agreed on the transfer with Real Madrid.
But they state on their website that a medical examination and details of the contract remain before the transfer is fully completed.