The baptistery, in Santa Maria Maggiore, was designed by the architect Flaminio Ponzio (1560-1613) in 1605, as part of a programme of additions to the basilica ordered by Pope Paul V (r. 1605-21).
The baptistery was originally the Chapel of the Winter Choir, where the canons celebrated the Divine Office in winter. However, in 1825 Pope Leo XII (r. 1823-29) ordered the canons to move out and commissioned Giuseppe Valadier (1762-1839) to convert the chapel into a baptistery.
The baptistery comprises two bays: an antechamber and the baptismal chamber. The richly decorated vault of the latter has frescoes by Il Passignano (1559-1638), which depict the Immaculate Conception with Prophets and Doctors of the Church. The altarpiece is a marble bas-relief of The Assumption of the Virgin by Pietro Bernini (1562-1629), the father of the much more illustrious sculptor Gian Lorenzo Bernini.
The font is an ancient porphyry basin. The gilded bronze statue of St John the Baptist was executed by Giuseppe Spagna to a design by Valadier.
There is a very fine bust of Pope Benedict XIII (r. 1721-24), the work of the Roman sculptor Pietro Bracci (1700-73), on the right side wall.