The Brazil international, 22, has signed a five-year contract with United after they agreed a deal with Premier League rivals Chelsea last week.
Santos made 43 appearances for Chelsea last season and has already started training at Old Trafford having been left out of Brazil’s World Cup squad.
Manchester United turned to Santos after missing out on two key targets in Elliot Anderson and Mateus Fernandes, who joined Manchester City and Tottenham respectively.
Exclusive analysis of England v Norway
Make sense of the drama with Metro’s free daily newsletter. Sign up now.
Santos is delighted to join ‘special’ United and believes Michael Carrick is the ‘perfect coach’ to take his game to another level.
‘Everything about Manchester United is special; it is an incredible feeling to join a club that some of my biggest idols have represented,’ he said.
‘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.
Santos and Man Utd transfer chief Jason Wilcox (Picture: Getty)
‘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.’
Jason Wilcox, director of football at Old Trafford, added: ‘Andrey is an outstanding midfielder with excellent technical qualities and the ability to impact the game at both ends of the pitch.
‘He was a key target for us in an important position, so we are delighted that he will join Michael’s squad so early in pre-season.
‘Despite already having extensive experience, leadership abilities and proving his talent at the highest level, Andrey still has enormous potential to develop further, and we cannot wait to see him flourish at Manchester United.’
Chelsea ‘thanked’ Santos for his time at the club and ‘wished him well for the next stage of his career’.
