The Catacombs of San Gennaro (St. Januarius) are considered by many to be the most significant paleo-Christian ruins in Italy south of Rome. They are situated in the northern part of the city, on the slope leading up to Capodimonte, on a site that is now easily identified by the large church of Madre del Buon Consiglio. In early Christian days, there were actually three cemeteries, dedicated, respectively, to San Gaudioso, San Severo, and San Gennaro. The three grew together and are now known collectively as the Catacombs of San Gennaro.

These Catacombs in Naples are different from their Roman counterparts in that they have more spacious passageways along two levels. The lower level is the oldest, going back to the third or fourth century and may actually be the site of an earlier pre-Christian cemetery later ceded to the new sect.

It became an important religious burial site only after the entombment of Bishop Agrippinus of Naples. The second level was the one expanded so as to encompass the other two adjacent cemeteries. The site was consecrated to Gennaro (Januarius) in the fifth century on the occasion of the entombment of his earthly remains there. The remains were later moved to the nearby Cathedral of Naples, where they are still housed.

Between the 13th and 18th centuries, however, the catacombs were the victim of severe looting. Restoration of the catacombs was made possible only after the transfer of skeletal remains to another cemetery.

Today the catacombs are prized especially for both the length of the period they were in continued use—from the emergence of Christianity until the 10th century—and the well-preserved fresco cycles that decorate their corridors and chapels, spanning from the second to the 10th centuries.