Santa Costanza is a 4th-century church in Rome, Italy, on the Via Nomentana, which runs north-east out of the city. It is a round building with well preserved original layout and mosaics. It has been built adjacent to a horseshoe-shaped church, now in ruins, which has been identified as the initial 4th-century cemeterial basilica of Saint Agnes. Santa Costanza and the old Saint Agnes were both constructed over the earlier catacombs in which Saint Agnes is believed to be buried.
According to the traditional view, Santa Costanza was built under Constantine I as a mausoleum for his daughter Constantina, later also known as Constantia or Costanza, who died in AD 354. However, more recent excavations seem to date the existing church to the time of Emperor Julian, who would have built it as a funerary structure for his wife, Helena, who died in AD 360, and was herself also a daughter of Emperor Constantine. If that should be the case, then the building was mainly dedicated to Helena and only secondarily to her sister Constantina, whose sarcophagus was brought into the new building from an earlier structure.