Casemiro said: “The opportunity to start a new challenge at Manchester United and in the Premier League is extremely exciting.
“I have worked with many great managers in my career and, having met with Erik and heard his ideas, I can’t wait to work closely with him, his staff and my new team-mates in the coming years.
“I’m ending one beautiful journey in Madrid whilst starting another in Manchester as determined as ever to win football matches, win trophies and make our fans proud by bringing success to this great club.
“Everybody knows the history of Manchester United, the significance of the club around the world and what it means to its supporters.