The former Liverpool and Panathinaikos winger joins the Serie A side on a free and will meet up with his new club in July
The Argentine attacker had been playing for Panathinaikos in Greece who he joined in 2009-10 and won the Greek Super League that season.
However suffering from a series of injuries he was released from the club in January and was without an employer when Catania secured his services.
Leto began his career in his homeland with Buenos Aries club Lanus before a move to Premier League side Liverpool.
He never made a league appearance due to work permit issues but played in the League Cup and Champions League.
Leto was loaned to Olympiakos where he won the Greek double in 2008-09 before his move to Panathinaikos.