The big striker has signed a five-year deal at the Turin club.
The 25-year-old made only 19 appearances for City and he was loaned out to Lazio for the second half of last season.
Bianchi was delighted to seal his return to Italy after an unhappy time in England.
"I am happy and aware I am going into a tough challenge, but a very, very exciting one," Bianchi told his official website.
"My only objective is to take Toro to the top along with my team-mates and the splendid fans."