The Baptistry was founded in 1152, but it took more than two centuries to complete. The first stage of construction, up to the top of the first row of arches, was directed by the architect Diotisalvi. At the beginning of the 13th century Guidetto took over, and supervised the ornamentation of the doors and walls, up to the top of the circular aisle. After 1260, Nicola Pisano worked on the outside wall, up to the level of the archlets in the external gallery, but it was Giovanni Pisano who added the pediments above.

The construction adapted to to the proportions of the Cathedral, with which he preserved a perfect axiality: the largeness of the facade exactly corresponds to distance between its main portal and the west door. 

The Opera di San Giovanni was instituted for supervising the new building yard and at first the citizens were asked to contribute with a one denaro tax pe month, ensuring a good start out of the works. The first Operaio, Conetto, brought back from the squarries of the Isle of Elba and of Sardinia eight columns and four pilasters for the bearing structure of the building, erected "with great triumph" in 1163. In these very years, the construction of the walls of the whole city, until then without any, started in the area next to the square. 

Work was then suspended for a long time, beginning once more in the mid 1300s, when the women’s gallery was completed and the baptistery roofed over. At the bottom, the outside of this circular building presents a row of round dead arches, broken by slit windows and four doors and supported by pillars against the walls. Above these is a gallery of arched columns with ornately carved pediments. Empty spaces in this gallery are due to some statues being transferred to the nearby Opera del Duomo Museum. These arches are decorated with human heads carved by Nicola Pisano and his workshop, in and after 1265.

Above, rises the dome, skirted by a row of triangular fronted marble aedicules completed with a pair of pinnacles. At the top is a statue in gilded copper of St. John the Baptist, made by Turino di Sano da Siena in 1395. The door facing the Cathedral was decorated in about 1203-1204 by sculptors of Greek-Byzantine culture; the cycle is flanked by two columns carved in spirals of foliage.