洗礼堂平面呈圆形，直径39米，圆顶上立有”施洗者约翰”的铜像。在入口处有各种富有教诲意义的圣经故事。教堂有3个主要入口分别通向中殿及通廊，其上则为4层开放的游廊。

The Baptistery, dedicated to St. John the Baptist，is the largest baptistery in Italy. Its circumference measures 107.25 m. Taking into account the statue of St. John the Baptist on top of the dome. The round Romanesque building was begun in the mid 12th century