The chapel immediately to the left of the entrance is the baptistry, and is appropriately dedicated to St John the Baptist. It was founded in 1479 by Giovanni di Montemirabile, who was a bishop of Vaison in France. The chapel has two ciboria by Andrea Bregno, which came from the former high altar. The baptismal font, to the left of the altar, and the receptacle for the holy oils, to the right, are adorned with handsome Cinquecento reliefs.