The Serbia international had already played for the Viola in 2011-12, scoring two goals in 29 competitive appearances for the club, before moving to Manchester City for €15.2 million.
Following Schalke 04’s relegation, Nastasic was released for a minimal fee to re-join his former side.
“I am very happy to be back here nine years since I left for England,” Nastasic told the official Fiorentina website.
“I was in love with this city and am so happy to return with my family. Clearly, the players have changed, but I still know some people in the staff and it’s wonderful to see them again.”
Nastasic comes as a replacement for departed captain German Pezzella, who has also completed his transfer to re-join former side Real Betis this week for a reported €4 million.
Nastasic revealed he had a conversation with his fellow countryman Nikola Milenkovic before moving to Stadio Artemio Franchi.
“I spoke to Nikola, he told me how things work here nowadays and I am ready to help the team. Forza Viola!”
Nastasic will have to wait for his debut, as he was not included in the match-day against Roma on Sunday.
Fiorentina having finished 13th last season in the league log expect to do better for the new season.
