The 30-year-old goalkeeper, who has won 17 caps for the Azzurri, returns to Serie A five years after leaving Palermo for the French capital.
Sirigu's arrival in Turin comes after the club opted not to pursue a permanent deal for Joe Hart, who spent last season on loan from Manchester City.
"I am very happy to be part of this club. I chose Toro because they wanted me strongly and have a sound project in place to grow over the coming years," he told the club's official website.
"The club and the president made me feel important and I thank them for that. I am honoured to be part of a club that has a unique story."
Club president Urbano Cairo added: "Salvatore Sirigu is a goalkeeper of international calibre, with considerable experience in European and world competitions and an enviable list of honours.
"He is 30 years old, in the prime of his career for a goalkeeper, and still has many years ahead of him as a leader. We are pleased to warmly welcome Salvatore Sirigu."
Sirigu won four Ligue 1 titles, two Coupes de France and three Coupes de la Ligue with PSG but lost his place to Kevin Trapp last season, spending much of the campaign on loan at Spanish sides Sevilla and Osasuna.