He was available on a free transfer after leaving St Etienne last month with 25 goals in 95 matches.
"I am very happy to have signed and to play here," said Ilan, who has also won three caps for Brazil. "I spoke to Gianfranco Zola and he is a very nice guy. I spoke a little Italian and a little French to him. I am happy to be here and it is a great opportunity for me.
"West Ham are a big club in England and a very traditional club. I also love the Premier League."
The arrival of three more forwards takes the number of strikers at Upton Park to nine as they desperately try to avoid relegation.
The Hammers lie one point off the bottom three, 15th in the table.
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