The 12th-century cathedral was built in the vicinity of a sacred building for the celebration of the sacrament of baptism, dedicated to Saint John the Baptist, which dates back to 1123. The interior of this Romanesque baptistery is separated into three naves and features a single large semicircular apse. The baptismal font, which dates back to the late 12th century, is octagonal in shape and decorated with eight scenes from the Gospel. It is believed to have been created during the early period of the sculptor Brioloto, who also created the rose window on the façade of the Basilica of San Zeno.
The baptistery contains frescoes from the 13th and 14th centuries, as well as other works of great artistic interest, such as a processional cross by Giovanni Badile (first half of the 15th century), a Madonna and Child with Saints on wood by Giovanni Francesco Caroto, dated 1514, and The Baptism of Christ, painted by Paolo Farinati in 1568.