The 26-year-old, who joins the Hammers for an undisclosed fee, has signed a five-year contract with the Premier League club.
Diamanti told West Ham's website: "I am very happy to be at West Ham United and to get the chance to play for Gianfranco Zola."
Zola has been keen to land a striker to partner Carlton Cole while Dean Ashton struggles to return from injury.
England forward Cole is West Ham's only fit senior striker at present.
Diamanti added: "I chose West Ham because the club have the tradition of playing great football. Also because the West Ham fans are really passionate and that is the way I am."
West Ham's chief executive Scott Duxbury said: "I am delighted we have been able to secure this transfer at last as we feel that Alessandro will be a perfect complement to our existing attacking options."
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