Bagai was a Roman–Berber city in the province of Africa Proconsularis. It must have been of some reasonable size, as it was also the seat of an ancient Catholic bishopric. The ancient city has been identified with ruins at Ksar-Bagaï outside of Baghai, in the Aurès Mountains of the El Hamma District in Khenchela Province, Algeria.
Bagai is a town in Numidia between the Aurès Mountains in the south and the salt lake Garaat al-Tarf in the north.
6 kilometers to the south of Bagai is Hammam Essalihine the ruins of an ancient Roman bath that dates from the time of the Flavian dynasty. The town of Aquae Flavianae was also near these ruins.
In the 2nd century, the city possessed monuments and dedications to the emperor Verus and to the emperor Marcus Aurelius.
Around 384, Donatist schism caused a conflict in the region. Emperor Constant I sent emessaries to reconcile the two religious parties, but the dissent lead to open revolt. The massacure of Bagai resulted. The city was a stronghold of Donatism and in 411, a Donatist bishop was sent from Bagai to the Council of Carthage.
The city was the most important capital in the conflict between Donatists and Catholics during the Late Empire. In the election of Primianus of Carthage, the region burst into flames again. The Berbers destroy the city and ransack the library.