The defender becomes the club's third high-profile signing of the summer on the same afternoon as Antonio Conte revealed that Emanuele Giaccherini will join Sunderland
The 25-year-old is one of the most highly rated defenders in Italy and was linked with Premier League champions Manchester United before the Bianconeri firmed up their interest.
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Since then he has blossomed into one of the Granata's star men and has built a reputation as an athletically blessed performer, who can cover both the centre-back and left-back slots.
Ogbonna made 10 appearances for Italy Under-21s side before progressing to the senior ranks and was included in the Azzurri's Euro 2012 squad, though he did not feature in the tournament.
His arrival comes on the same afternoon on which it was revealed that Emanuele Giaccherini will leave Juventus Stadium for Sunderland, as the Serie A champions look to claw back some of their transfer outlay.
Strikers Fernando Llorente and Carlos Tevez have already joined the club in what has been a busy summer transfer window for the Bianconeri so far.