The 26-year-old has put pen-to-paper on a five-year contract.
But rather than testing his skills in Italy's top division, the set-piece specialist has opted for the Premier League.
"This is a big club and I want to give everything for the supporters."
Diamanti, who will wear the No32 shirt, added: "I chose West Ham because the club has the tradition of playing great football.
"Also because the West Ham fans are really passionate and that is the way I am."
Hammers CEO Scott Duxbury said: "I am delighted that 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."