Released for a nominal fee by Schalke, the defender has returned to his former Serie A club.
“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 admits he spoke to Serbia teammate Nikola Milenkovic before committing to the move.
He added: “I spoke to Nikola, he told me how things work here nowadays and I am ready to help the team. Forza Viola!"
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