German Nsereko arrives on a four-and-a half-year contract.
The 19-year-old forward, who can play wide or through the middle, chose the chance to play under Gianfranco Zola ahead of long-standing interest from some of the continent's leading clubs. The fee is undisclosed, although the club confirm it is a significant investment.
West Ham have handed Savio the No10 shirt and CEO Scott Duxbury told the club's website: "We are delighted to sign Savio, particularly with the fierce competition for his services across Europe. He is an exciting prospect, who Gianfranco Zola believes will prove a valuable asset for this club now and in the years to come.
"It is a significant investment and shows our determination to continue moving this club forward."