Bah, who spent the second-half of last season on-loan with Lens, has joined Nice on a 12 month temporary deal. Nice count on Manchester United minority shareholder Sir Jim Ratcliffe as their owner.
Bah joined City in January after they paid his buyout clause - a move that left former club Real Valladolid infuriated.
Nice sporting director Florian Maurice said: "We have been following his progress for a long time and we found him to be a very good player during his time at Lens.
"The numbers prove it. Juma is a young man with great potential and, importantly, we think he can adapt quickly to the coach's ideas."
Rosenior a key
Bah also said: "A team that I know well from my experience in Ligue 1. There is quality and experience in every department. The coach (Liam Rosenior) has very precise and offensive ideas, which all the players follow, including the defenders.
"This was an important factor in my choice. I am proud to join this club and this project.
"Important dates await us at the beginning of the summer. It will be an exciting season. It's up to me and us to be ready!"