The Catacombs of San Sebastiano are a hypogeum cemetery in Rome (Italy), rising along Via Appia Antica, in the Ardeatino Quarter. They are one of the very few Christian burial places that have always been accessible. The first of the former four floors is now almost completely destroyed.

Thanks to the excavations carried out at the end of 19th and during the 20th century, it was possible to recreate the topographic and architectural history of the area - consisting of three levels of galleries - in which the catacombs lie.

The area used to be a pozzolan mine; it was abandoned at the end of the 2nd century and then used by Romans as a place for pagan burial: simple graves for slaves and freedmen have been discovered, as well as monumental tombs, particularly in the so-called piazzola ("little square"), a circular compartment that had been an opencast mine, in which walls three mausoleums were dug. The presence, in these mausoleums and particularly in the so-called Mausoleum of Innocentiores, of typically Christian iconographies, such as the anchor and the fish, suggests that the mausoleums were used, at a later stage, also for the sepulture of Christians. Beside the piazzola, the dig of the cemetery galleries was started in this period.

Around the half of the 3rd century the whole piazzola was filled in, so as to create an embankment at an upper level. Three monuments have been brought to light on this shelf: the so-called triclia, a covered porticoed hall used for burial banquets, whose walls display more than 600 graffiti with invocations to the Apostles Peter and Paul; a marble-upholstered aedicule that, according to archaeologists, was the place where the relics of the two Apostles were kept during the period when they were moved in catacumbas; and a covered room with a well to draw water. The transfer of the relics of the Apostles to San Sebastiano in the mid-3rd century and their relocation in the former places at the beginning of the 4th century is still a debated issue among researchers and archaeologists.

Finally, in the first half of the 4th century also these spaces were buried, in order to build the embankment on which the Constantinian basilica was erected.