Like the cathedral and the campanile the Baptistery is built of bichromatic Carrara marble, white with recurring horizontal lines in blueish-grey stone, also used for abstract floral and graphic decoration, a unique trait of some of the most important religious buildings in Tuscany (In the neighboring Florence and Pistoia the dark marmo verde from Prato was used).

On the right it begins at the top with the Ascension of Christ, then angels, Mary with lifted hands, then the Apostels depicted in pairs looking up, and second to the bottom the Harrowing of Hell; the lowermost relief shows King David.

The tripartite form is conveyed in the arch with three retreating archivolts with the Twenty-Four Elders in medaillons and the Lamb as the keystone.

[2] Only the north portal has also figurative decoration on its architrave, picturing the Annunciation to Zechariah and St. Elizabeth, the parents of St. John, flanked by two prophets and two angels in light armour with swords.

The scenes on the pulpit, and especially the classical form of the nude Hercules, show Nicola Pisano's qualities as the most important precursor of Italian Renaissance sculpture by reinstating antique representations: surveys of the Italian Renaissance often begin with the year 1260, the year that Nicola Pisano dated this pulpit.

As a side effect of the two roofs, the pyramidal inner one and the domed external one, the interior is acoustically perfect,[3] making of that space a resonating chamber.