Andre Villas-Boas' men have completed the signing of the Argentina international from Benfica on a five-year deal for a reported fee of €15m
The centre-back has attracted the attention of several clubs after an excellent season with the Portuguese giants in which they won the Europa League.
And the former Real Madrid player has secured a move to the Russian side, having joined on a five-year deal active from July 1 for a fee reported to be around €15 million.
"Football clubs Zenit and Benfica have reached an agreement over the transfer of the defender of the Argentina national team," a statement on the Russian side's official website read. "The contract has been signed for five years from July 1. The player will join the team after the World Cup."
Benfica also confirmed the deal, revealing that they will make €6m from the sale, however the true transfer fee is reported to be around €15m as the Portuguese club owned only 40 per cent of the player, while Real Madrid owned 50% with the remainder belonging to a third party.
"Benfica announces that it has reached agreement with Zenit to transfer outright the sporting and economic rights of Ezequiel Marcelo Garay Gonzalez for the amount of €6m," the club announced.
Garay thanked coach Jorge Jesus and his team-mates for the support they have shown him in the three years he spent in Portugal.
"I want to thank everyone who supported me in these three magnificent years I spent at Benfica," he told Benfica TV.
"In particular the president, who believed and invested in me, and the coach who made me evolve and reach a level that I'm very proud of.
"Thanks also to all my team-mates with whom I had the honour of playing with for this club.
"I want to return all the love that Benfica and the supporters gave me. I'll take all the unforgettable moments I've had in Portugal in my heart. Thanks!"