The Baptistery of Albenga is known as the most distinguished late antique monument in north-western Italy, which has maintained its ancient baptismal functions to this day.
It rises next to the left side of the Cathedral and with it constitutes the original episcopal nucleus; its construction is perhaps contemporary to that of the church and attributed today, pending more precise stratigraphic indications, between the last quarter of the V and the first years of the VI century; and this following the reconstruction of the city and the organization of its civil and ecclesiastical territory, due to Constantius, then emperor Constantius III, starting from 417 d. C .; The existence of the Diocese is also documented by the first sure citation of a bishop, Quintius who signs in a meeting of bishops in Milan, an archdiocese of which the diocese of Albenga remains a suffragan until the beginning of the 13th century.
The building, today at a level of about m. 2.50 lower than the current one, it has an octagonal plan on the inside and decagonal on the outside, perhaps to adapt to a previous urban situation. It is a classic late antique building, with the interior marked by eight niches, with a square and semicircular plan: from the eight sides of the ground floor to the sixteen sides of the upper floor, with windows alternately closed and open; from these we passed to the dome, the remains of which were removed in the late nineteenth century restoration by Alfredo D'Andrade, who also saved the monument; the dome, proven by the presence in the structure of numerous amphorae still in situ , was replaced by a wooden roof.
The interior shows the original structure almost intact, which has an octagonal basin in the center still covered with marble slabs and an external star structure; here, raising the bottom of the basin, the new baptismal font built according to the norms of the Council of Trent was positioned by Bishop Luca Fieschi (circa 1585-90); source now positioned in one of the side apses. The interior of the Baptistery, however, retains a series of decorative elements of the highest level. This is in particular the mosaic decoration of the apse in front of the entrance, with the monogram of Christ three times depicted with splendid chromatic accents in the vault, to remember the Trinitarian and Christological dogma, surrounded by twelve flying doves, directed towards a small cross; in the back lunette two lambs are depicted on a flowery meadow, next to a large jeweled cross. The vault of the apsidiole is surrounded by a band of acanthus leaves; while below there is a list of saints from the Roman and Ambrosian tradition. Other furnishings of great value are the stone ones, both the two from the V-VI century, and in particular the large perforated early medieval slabs, in sandstone, which close some single-lancet windows, attributed to the first half of the VIII century, at the time of the Lombard king Liutprand. Three marble slabs belong to the same period, a fragmentary slab which acts in part as a lid and the lunette inserted in the arch that form the complex of the tomb to the right of the entrance; these plates, with the exception of the lunette still in situ , are now replaced by casts and the originals preserved in the Diocesan Museum. They are all pieces that bear a splendid workmanship; the large slab, attributed to Hispano-Visigothic workers, has a dense decoration of three-ribbon braids, which enclose daisies, lily elements, leaves, bunches of grapes, a pecking bird; in the lunette stands a large flowered branch flanked by racemes and motifs of circles and lozenges, while the partial cover plate bears a large Greek cross with expanded arms, with rows of pearls, daisies, pinwheel rosettes and lilies.
Still some pictorial decorations can be assigned to the late Middle Ages, and the documentation certifies the continuity of the baptismal function of the building; suspended in the nineteenth century and after the restoration of D'Andrade, it was resumed starting in 1994.