Zenit, which finished third in the Russian Premier League, said in a statement the club had signed the 52-year-old Italian on a three-year contract with the possibility of a two-year extension.
A source familiar with the contract told Reuters that Mancini would be making 4.5 million euros (£3.9 million) a year.
Mancini is one of the highest-profile foreign coaches to join the Russian Premier League in recent years.
Zenit, which is bankrolled by Russian energy giant Gazprom, has used its deep pockets to attract pricey foreign managers in the past, including Dick Advocaat, Luciano Spalletti and Andre Villas-Boas.
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Mancini has coached a number of elite sides including Inter Milan, Galatasaray and Manchester City.
In 2012 Mancini led Man City to the club's first English Premier League title in 44 years.
He parted ways with Inter Milan last summer after two seasons as head coach, ending his second stint as manager of the Serie A side.
Zenit said last week it had fired Romanian trainer Mircea Lucescu after one season with the club, without providing a reason for his dismissal.
(Reporting by Dmitry Zhdannikov and Moscow newsroom Editing by Jeremy Gaunt)
