Santa Costanza is an imperial mausoleum in Rome that was later dedicated as a church (in Italian it is known both as Mausoleo di Santa Costanza and Chiesa di Santa Costanza). This fascinating and beautiful building dates from the 4th century AD and features some of the earliest surviving Christian art.

The Mausoleum of Santa Costanza is named for Constantine the Great's daughter Constantia (also known as Constantina or Costanza), but the latest scholarship suggests it was actually built for her sister Helena, who died in 360-61.

Early accounts (such as the Liber Pontificalis) record that Constantine built a funerary hall here on the imperial estate at the request of Constantia - this is the long building now in ruins next to the mausoleum. The funerary hall was dedicated to the virgin martyr St. Agnes and resembled others built by Constantine in this period.

The early accounts also report that Constantine built a baptistery on the site, in which both Constantia and Constantine's sister were baptized by Pope Sylvester (314-35).

Construction on the funerary hall and baptistery likely began after 326 (Constantine's last visit to Rome) and must have been completed by 329-330 (the death of Constantine's sister, who is known to have been baptized there).

It was long assumed that the existing mausoleum is this baptistery, but excavations beneath its north end in 1992 revealed a triconch (clover-shaped) building that is a more likely candidate.

After Constantia died in 354, her body was brought back from Bithynia to be buried on the imperial estate in Rome (Ammianus, History 14.11.6).

It was long thought she was buried in the mausoleum, but new data indicates the building was probably still unfinished at that time. So she was probably buried either in the baptistery or in the apse of the funerary hall.

The round mausoleum that survives today was probably built in the 360s or 370s, based on the combination of pagan and Christian elements in the mosaics.