The Church of Santa Costanza is an Early Christian rotunda decorated with mosaics dating to the 4th century. Traditionally identified as the mausoleum of Constantina, the daughter of Constantine, is a noteworthy example of Early Christian art and architecture. Its mosaic decorations are also important in showing how Early Christian art combined pagan and Christian elements.

Constantina's villa was located on the Via Nomentana around 3 kilometers outside the walls of Rome. It was the site of the catacombs of St. Agnes, and later a church complex consisting of a baptistery and the adjoin basilica of St. Agnes to the west of the catacombs in which St. Agnes was buried. It seems that Constantine was responsible for the construction of the basilica of St. Agnes, built at the request of his daughter, Constantina, on property she owned near the tomb of the saint. It is possible that both were the patrons, with Constantine providing the initial funding and Constantina overseeing the work. The date of the complex is uncertain, though it is possible that it was completed by 335. When Constantina, daughter of Emperor Constantine, died in Bithynia in 354, her body was returned to be buried at an imperial a mausoleum at the villa. Later her sister, Helena, wife of Julian the Apostate, was also buried here after she died in Gaul in 360 or early in 361. Archaeological evidence makes it clear that the mausoleum was completed at a later date. It is unclear whether Constantina had the mausoleum built or whether it was built later, perhaps to house the body of her sister, Helena. At a later date, the mausoleum became the Church of Santa Costanza.

The mausoleum, a brick rotunda measuring around 29 m in diameter, was once attached to the Basilica of St. Agnes on its south side near the entrance. This brick basilica, which measured around 120 meters in length, is now mostly in ruins. Much of the apse wall of the brick basilica remains, but the narthex has almost completely disappeared.  The core of the rotunda consists of a domed circular chamber 11.5 m. in diameter, separated from a ring-shaped ambulatory by twelve pairs of columns. It has a unique order of columns, with two columns, comprised of Roman Attic bases, shafts of gray and red granite and composite capitals, sharing a common entablature. In addition, its dome has twelve windows. Across from the door there is a large niche above which a small turret rises; this was the porphyry sarcophagus until the 18th century. It is possible the two smaller niches on the cross axis once held sarcophagi as well. Another porphyry sarcophagus, removed from the mausoleum in 1606, now holds the relics of St. Simon and St. Jude in St. Peter’s Basilica in the Vatican.  

Archaeological evidence has shown that the rotunda was built over an older structure that was built at the same time as the basilica. This building, which seems to have been a triconch that was destroyed in order to build the mausoleum, might have been the original mausoleum, a martyrium for the veneration of St. Agnes or perhaps the baptistery Constantina had built. Excavations at the basilica in 1956 uncovered the foundations of a unique apsed structure in the middle of the nave of the cemetery basilica, which could also have been the original mausoleum of Constantina. It is possible, as Constantina was probably quite young when construction of the complex started, that the mausoleum was not part of the original complex. It is also possible that this mausoleum did not even exist at her death and her body was placed elsewhere, perhaps the triconch, even if it had previously served as a baptistery, and only at a later date her body moved to the mausoleum.  Tearing down an imperial building, like the triconch, and building a more splendid mausoleum would require a powerful patron. While Constantina is often considered the patron of the mausoleum, it has also been argued that it was built for Constantina's younger sister Helena.

The mausoleum was once elaborately decorated, which, though mostly lost, can still be seen in its vaults and niches. Drawings from the Renaissance record some of the lost decoration. Originally the cupola, vaults and niches were covered with mosaics, while the drum over the arcade was decorated with opus sectile simulations of pilasters, cornices, and other architectural motifs.  The mosaic decorations, long been noted for the combination of mythic, secular and Christian imagery, mark the emerging iconography of the Christian faith in its first years of legitimacy in the Roman Empire. Scenes on the cupola of S. Costanza included a silver mosaic river, a marine scene in which cupids fished, sailed, speared octopus with tridents, and panthers of Bacchus flanking female caryatids as well as many Christian representations, including scenes from the Old and New Testaments. The ambulatory was decorated with cupids, nymphs, and scenes of vintage, while images of Christ were in the tower and niches. Furthermore its windows and columns have also been considered to symbolize the 12 apostles, while its axial niches mark a cross. Even the pagan elements could have Christian significance. For example, Dionysiac themes, which appear in both the mosaics and sarcophagus, were also frequently employed in Early Christian art to symbolize both the Eucharist and resurrection.

There is an important early depiction of the traditio legis, Christ giving the law, in one of the apses. Christ has his right arm raised in a gesture of address and his left arm holds an open scroll, as Peter approaches from the right and Paul acclaims him on his left. Below Christ are lambs, which represent the faithful who approach the Lamb of God in paradise. They issue from Jerusalem and Bethlehem, which along with St. Peter and St. Paul, represent the Jewish and Gentile churches. It emphasized the primacy of Peter and the importance of Paul as the chief apostles and as Roman martyrs, though it is primarily eschatological, revealing Christ as the resurrected Lord of the Second Coming. The traditio legis first appeared in the 4th century in Rome, perhaps first for the apse of St. Peter’s Basilica. Another apse mosaic shows Christ bearded, with a nimbus, and in purple imperial robes, seated on the globe of the cosmos, as he presents the key of His Kingdom to St. Peter.