The Lateran Baptistery (Italian: Battistero lateranense, also known as San Giovanni in Fonte or San Giovanni in Onda) stands apart from the Archbasilica of Saint John Lateran, Rome, to which it has become joined by later construction.

[1] This baptistry was for many generations the only baptistery in Rome, and its domed octagonal structure, centered upon the large octagonal basin for full immersions, provided a model for others throughout Italy, and even an iconic motif of illuminated manuscripts, "The fountain of Life".

Attached to one side, towards the Lateran Basilica, is a fine porch with two porphyry columns and richly carved capitals, bases and entablatures.

While its interior architecture was consolidated and embellished after plans of Gian Lorenzo Bernini around 1633,[2] a fresco cycle with scenes from the life of Constantine was added by Andrea Sacchi on the walls of the ambulatory.

[3] Its plain brick exterior was embellished with a frieze designed by Francesco Borromini in the late 1650s or early 1660s, incorporating the arms of Pope Alexander VII.