The 31-year-old joined City last summer from Inter Milan but struggled to win a regular place in the first team, making only 11 appearances during last season, just four of those in the league.
The move back to Serie A was confirmed on City's website and the club said: "Everyone at Manchester City wishes Maicon good luck at his new club."
Maicon found competition from Pablo Zabaleta and Micah Richards for that right-back spot too strong.
New City boss Manuel Pellegrini said at the weekend that Maicon was not part of his plans and the Brazilian did not take part in the pre-season tour of South Africa.
Pellegrini said: "Maicon didn't come here to the pre-season and he will not be in the squad at Manchester City this year."