Castrogiovanni, the Premiership Player of the Year in 2006/07 in his first season with the club, made 146 appearances for Leicester Tigers.
The prop, who has 95 caps for Italy, made his last appearance for Tigers in the Aviva Premiership Final, collecting a fourth league title in his seven years with the club.
His move to Toulon was confirmed by the Heineken Cup winners on Friday.
Speaking after the Aviva Premiership Final, Tigers director of rugby Richard Cockerill said: "I've got nothing but respect for Castro and what he has done for this club.
"He has had seven years here, he's a great bloke, a great player and we will miss him. He will be remembered as one of the iconic players in the club's history.
"He is 31 and wants to play as much as he can. We understand that and we wish him well for the future. Now he leaves as a Champion of England and he will always be welcome at Welford Road."