The Baptistry is the beautiful octagonal building opposite the cathedral, and once the cathedral itself. It dates from the thirteenth century. It is famous for the bronze doors by Ghiberti called the "Gates of Paradise". These gilded bronze doors consist of panels depicting a biblical scene from the New Testament. The panels are surrounded by a framework of foliage in the doorcase and gilded busts of prophets and sibyls at the intersections of the panels. On the East side are doors depicting scenes from the Old Testament

The Baptistry is rich in color both without and within. The floor alone is a marvel of intricate inlaying, including the signs of the zodiac.  It is crowned by a magnificent mosaic ceiling. This mosaic cycle depicts in the three sections above the high altar, the Last Judgment with a gigantic and primitive Christ and the Angels of Judgment at each side by Coppo da Marcovaldo.    The other scenes on the ceiling depict different stories.

The most notable sculpture in the Baptistery is the tomb of the ex-Pope John XXIII. The tomb is exceedingly fine, the work of Donatello and his partner Michelozzo.