Begun on August 15, 1152 under the direction of Pisan architect Diotisalvi, this masterpiece of Romanesque architecture was enlarged on several occasions: in the course of the 13th century by Nicola and Giovanni Pisano, in the 14th century and, finally, in the 19th century when member of the Primaziale of Pisa, Vincenzo Carmignani, assisted by Pisan architect Alessandro Gherardesca, began a series of very contested restorations.

The Baptistery is also a "Galilean site": on February 19, 1564 Galileo was baptised here. The baptismal record can be read in the Libro del battesimo al Duomo, Lettera C (1564-1568), c. 36v, 221: «Galileo of Vincentio Ghalilei from Florence and Madame Giulia his wife was baptised on the 19th of the aforementioned day; sponsors and knights were Messrs. Pompeo and Haverardo de Medici, in the Chapel of Sant’Andrea».