The Brazilian midfielder, who has committed to a four-year contract at Old Trafford, will cost United up to £70 million.
After giving Real a tearful goodbye, Casemiro claims he is joining “the greatest squad in the world.”
The 30-year-old rises to the top earners at Old Trafford, but he insists that his decision was not driven by financial gain.
After working at the Bernabeu for nine years, he said that he would have left four or five years prior if it had been for the money.
“Those who believe that are incorrect; they have no idea who I am.”
“I’ve always been treated quite nicely by the club. So I choose this course of action.”
He said after being identified as a United player: “It’s thrilling to have the chance to embark on a new adventure at Manchester United and in the Premier League.
“I’ve had the pleasure of working with several outstanding managers. After meeting Erik and hearing about his plans, I can’t wait to collaborate closely with him, his team, and my new teammates in the coming years.
“I’m beginning a new adventure in Manchester after ending in Madrid, and I’m just as driven as ever to succeed in this great club by winning football games, trophies, and the admiration of our supporters.
“Everyone is familiar with Manchester United’s past, importance to the international soccer community, and relevance to its fans. It is an honour to represent United, and I am willing to do all it takes to assist the club reach its goals.”
Casemiro, who joined Real from Sao Paulo in 2013, assisted Real in winning the Copa del Rey, La Liga, the Champions League, and five other competitions.
He made his 336th appearance for the Spanish giants as a substitute in last week’s opening-day victory against Almeria. He started last season’s Champions League final as Real defeated Liverpool to win Europe for the 14th time.
On Monday, he said a tearful goodbye to the Spanish champions before departing for Manchester.
After the Champions League final, Casemiro revealed, “I talked to my agency since I got the sensation that I was approaching to the end of my cycle here.”
“The choice was the toughest of my life, but I’m looking forward to what lies ahead. I want to experience new difficulties and learn about various leagues, nations, and cultures.
I’m heading to the most extensive team in the world, which once rivaled Real Madrid’s glory but no longer did.
United, which has lost its opening two Premier League games, will play Liverpool on Monday at Old Trafford.
With Casemiro, Cristiano Ronaldo won four Champions League championships at Real, but this summer, the Portugal star has been heavily linked with a transfer away from Old Trafford.
Casemiro said, “I haven’t talked to Cristiano, but I hope he remains since he’s one of the greatest players of all time.
“I want to do there everything I did here. There, I want to win the league.
“I’m in the midst of one of my greatest phases professionally. I think I’m in terrific physical and mental shape, and I think now is a wonderful time to make this choice.”
Florentino Perez, president of Real Madrid, shed a few tears during the ceremony at the facility where the Spanish champions practice.
In all of these years here, “you have been exceptional every single day,” he stated. “We must respect your freedom to choose your future because you have earned it.
“You depart in the persona of the champion you are. You are visiting the storied Manchester United, a club you know well, but you will always represent Real Madrid wherever you go.
“You are one of us now and forever. Your home is here and always will be.”