The Sicilian club released a statement confirming the deal had been done with the player, who is out of contract.
Leto had also been linked with Torino and Fiorentina in recent months.
The 26-year-old had played for Lanus, Liverpool and Olympiacos.
He was signed by Liverpool in August 2007 for £1.85m, but never made his Premier League debut due to issues with his passport and had a work permit denied.
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