Ramires has put pen to paper and signed a four-year deal with Premier League champions Chelsea.
The Brazilian midfielder joins Carlo Ancelotti's side having received a work permit on Thursday and passed a medical.
Ramires, who arrives at Stamford Bridge for a reported £18 million from Benfica, will miss Chelsea's Premier League opener against West Bromwich Albion on Saturday and will link up with the rest of the squad next week.
The 23-year-old told Chelsea's official website: “Every player in the world would love to play for Chelsea and it’s a great opportunity that was given to me.
"I will give my best to repay this chance, and I am very happy."
Blues boss Carlo Ancelotti said: “He is a fantastic player, a fantastic midfielder, he is very young and will be the future of the Brazilian national team, and I hope he will be the future of Chelsea.”
Ramires helped Benfica to the Portuguese title last year and played in the first four of Brazil's World Cup matches before sitting out the quarter-final defeat to Holland having accumulated two yellow cards.