"The player's advisors and officials from both clubs have finalised the details over a four-year contract," City said Wednesday on its website www.mcfc.co.uk.
The transfer fee was undisclosed, but a reported deal of around 25million pounds was mentioned.
"Firstly I would like to thank Valencia, all of their loyal fans, my team-mates and the staff there," Silva, who is with Spain in the World Cup told City's official site.
"The time is right for me to seek a new challenge and I am thrilled about playing in England with Manchester City.
"I believe the Premier League is one of the best competitions in the world and I want to bring success to City and win trophies for them.
"There has been a lot of speculation about me in the British and Spanish media and I need to fully concentrate on playing for Spain in the World Cup.
"That is why my advisors have been dealing with City for me so that I can have my attention only on South Africa and playing well for my national team."