The 24-year-old, who is currently at the World Cup with his national squad, has agreed a five-year deal at Eastlands.
Silva will undergo a medical and the transfer will be formally completed once his involvement in South Africa is over.
Speaking from Spain's World Cup base, Silva told the club's website, www.mcfc.co.uk: "Firstly I would like to thank Valencia as a club and all of their loyal fans, and my team-mates there and staff.
"I have enjoyed six years at Valencia, they discovered me as a footballer and they will always be a special club for me.
"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 attentions only on South Africa and playing well for my national team."