The sacristy block, including the baptistry, was designed and built by Flaminio Ponzio in 1605 as part of his massive program of additions to the basilica ordered by Pope Paul V. The baptistry itself was originally the Chapel of the Winter Choir. In 1825 Pope Leo XII ordered Giuseppe Valadier to convert the old chapel into a baptistry.

Through the door in the left wall of the antechamber of the baptistry is the basilica gift shop. This is the former mediaeval Chapel of SS Michael and Peter in Chains, which was refitted and frescoed in the restoration by Cardinal Guillaume d'Estouteville (1443-83). The frescoes were rediscovered in 1913, having been hidden by a false ceiling after the 18th century transformations of the basilica by Fuga. The museum was inaugurated by Pope John Paul II in December, 2001.