The Church of Saint Susanna at the Baths of Diocletian () is a Roman Catholic parish church located on the Quirinal Hill in Rome, Italy. There has been a titular church associated to its site as far back as AD 280. The current church was rebuilt from 1585 to 1603 for a monastery of Cistercian nuns founded on the site in 1587, which still exists there. The church served as the national parish for residents of Rome from the United States from 1921 to 2017, during which period it was assigned to the care of the Paulist Fathers, a society of priests founded in the United States. The Paulist Fathers' ministry to United States Catholics subsequently moved to San Patrizio (Saint Patrick). Architectural history Roman era About AD 280, an early Christian house of worship was established on this site, which, like many of the earliest Christian meeting places, was in a house (domus ecclesiae). According to the 6th-century acta of Susanna, the domus belonged to two brothers named Caius and Gabinus, prominent Christians. Caius has been identified both with Pope Caius and with Caius the presbyter, who was a prefect and who is a source of information on early Christianity. Gabinus or Gabinius is the name given to the father of the semi-legendary Susanna of Rome. Her earliest documented attestations identify her as the patron of the church, not as a martyr, and previously the church was identified in the earliest, fourth-century documents by its title "of Gaius" by the Baths of Diocletian or as "ad duas domos" ("near the two houses"). It is mentioned in connection with a Roman synod of 499. The Church of Santa Susanna is one of the oldest titles in the city of Rome. The early Christian church, built on the remains of three Roman villas still visible beneath the monastery, was situated immediately outside the wall of the Baths built by Diocletian and the Servian Wall, the first walls built to defend the city. According to tradition, the church was erected on Susanna's house, where she was martyred. In the 4th century it was marked with the designation ad duas domos (at the two houses). This first three-aisled basilica was almost certainly built under the pontificate of Pope Leo III (795–816). According to tradition, the structure became a church around 330, under Emperor Constantine I, when the basilicas of numerous house churches came