The Spain international has agreed a five-year contract and cost Russia's Zenit 15 million euros (£11.99 million).
The 27-year-old started his career at Real Madrid but only made 18 league appearances for the European champions. He moved on to Osasuna and then Benfica, where he played alongside Axel Witsel, who also now plays for Zenit.
"The talks went on for along time," Garcia told the Zenit website.
"I am really happy to become a Zenit player and I want to start helping them as soon as possible.
"I spoke with (coach) Andre Villas-Boas a couple of times by telephone and he is one of the main reasons I came to St. Petersburg. Andre told me a lot about his project for the club and I think we can achieve a lot together."
Garcia made his Spain debut in 2012 and joined Manchester City the same year.
Garcia is the second major signing for last season's Russian league runners-up. Defender Ezequiel Garay, who played for Argentina at this year's World Cup in Brazil joined from Benfica on a five- year contract.
Zenit play Standard Liege in the final qualifying round for this season's Champions League.
(Reporting by Dmitriy Rogovitskiy, editing by Ed Osmond)
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