The Baptistère Saint-Jean is a religious edifice in Poitiers, France. It is reputed to be the oldest existing Christian building in France and one of the most prominent examples of Merovingian architecture. The central part of the building was constructed around 360, atop the substructures of Roman buildings that were demolished in 276, in what subsequently became the episcopal quarter of the town of Poitiers, near the residence of Saint Hilary and the future cathedral. Many changes have been made to the original structure. A baptismal tank was added in the 6th century, as at that time baptism was conducted by total immersion.
The building was badly damaged during the Visigoth occupation in the fifth century. A restoration was begun after Clovis I defeated Alaric II in 507 at the Battle of Vouillé, near Poitiers. This restoration included the repair of the upper parts of the walls of the cella, the addition of three apsidioles in the form of a transept and an apse, and the decoration of outside and inside of the building.