The 31-year-old defender returns to Italy on a loan deal after spending the last 18 months in Spain
After spending the last season and a half in Spain with Javier Aguirre's side, the 31-year-old defender will return to the Italian peninsula.
Contini was unveiled to the fans on Thursday and sporting director Giorgio Perinetti spoke of the club's newest arrival.
"Contini does not need much of an introduction. He wanted to return to Italy and to facilitate his arrival in Siena has made some personal sacrifices. I hope this turns out to be a long marriage," he said on the club's official website.
Contini has spent nearly his entire career in Italy, with three-year stints both at Parma and Napoli before moving to La Liga.
The financial details of the deal have not been disclosed.