The Baptistero has been called "perhaps the most beautiful Baptistery in Christendom." At 181 feet tall by 115 feet wide, it is still one of the largest in Italy. Like many Italian baptisteries, it was built apart from the church, probably because originally only baptized persons could enter a church. "Very simple in form, it is almost riotous in the luxuriance of its ornamentation," write Janet Ross and Nelly Erichsen in The Story of Pisa. It combines the best of Pisan-Romanesque and Gothic styles.

The baptistery's greatest treasure is a marble pulpit carved by Nicola Pisano in the 13th century. Pisans so prized the sculpture that, as soon as Pisano completed it in 1266, it was placed under the guardianship of the law. The baptistery also contains a marble font with eight faces, each decorated by central rose windows and geometric designs in marble. In the center of the font is a statue of St. John the Baptist crafted by Italo Griselli.