The 64-year-old has signed a three-year contract and joins the Hammers from Chinese side Hebei China Fortuna, who he left on May 20.
West Ham joint-chairman David Sullivan said he is delighted with the arrival of Pellegrini.
“It was important that we appointed someone with knowledge and experience of the Premier League, who already has an understanding – not only of the teams and players we face – but of West Ham United and our ambitions,” he told the club’s website.
“Manuel brings a reputation for attacking football and getting the best out of his players. We believe he will attract new talent to the London Stadium as well as improving the current squad.
“Above all, he is a winner, who knows what it takes to succeed at the highest level and is driven to continue that success. Manuel is the first West Ham United manager to have a Premier League title on his CV, and we believe that his experience, quality and proven record of taking teams forward quickly will ensure that he is successful here.
“We have listened to fans who asked us to be ambitious. We hope they agree it is an exciting appointment.
“Manuel will shortly announce the names of the new staff he will bring in and he will then get down to shaping the squad he wants. We all wish him the very best of luck.”