The 22-year-old has put pen to paper on a long-term contract to become a United player, committing to the club until 2031.
Reports state that United have paid £48 million for Santos in a deal which includes easily achievable £2 million in add-ons as well as a 10% sell-on fee for Chelsea.
Speaking on his move, Santos revealed that he is over the moon to sign for the Red Devils, who finished 4th last season under now permanent boss Michael Carrick.
“Everything about Manchester United is special; it is an incredible feeling to join a club that some of my biggest idols have represented.
“As a midfielder, I am really excited to have the opportunity to learn from Michael Carrick, he is the perfect coach to help me take the next step in my career and push to achieve my dreams.
“Everybody told me about how ambitious the club is and the amazing environment that has been created here. I know just how strong the squad is and I cannot wait to fight together to compete for the biggest trophies.”
Santos has made 47 appearances for the Blues in all competitions, scoring three goals and registering five assists. He was the first player to be signed under the Todd Boehly-Clearlake regime in July 2022 but has failed to impress in West London.
The youngster was not part of the Brazil squad for this summer’s World Cup and will reportedly join Carrick’s pre-season side over the coming weeks.