The 27-year-old has signed a four-year contract at Upton Park despite reported late interest from Premier League rivals Everton and Southampton.
Ogbonna is the second Serie A player to make the move to West Ham this summer, following Pedro Obiang’s transfer from Sampdoria last month.
“I am delighted to have joined the club,” he told the official website. “I wanted to play in the Premier League and am happy to be in London. My heart told me that the right decision was to play for West Ham United.
???I cannot wait to start working under Mr Bilic and with my new team-mates. I feel ready for this new adventure.???
Ogbonna has 10 caps for his country and spent his formative years in Torino’s system, going on to make 147 appearances before crossing Turin to join Juventus at the start of the 2013/14 season.
He made 41 appearances in his two seasons with Juve and celebrated a pair of Serie A titles, but he was an unused substitute when they lost the Champions League final to Barcelona.