The 64-year-old Chilean has taken over at London Stadium from David Moyes on a three-year deal.
West Ham's joint-chairman says that Pellegrini’s experience as manager of Manchester City, where he won the 2013/14 Premier League title, and his reputation for attacking football, were key factors behind his recruitment.
“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,” Sullivan told West Ham’s official website.
“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.”