Pope Caius, also called Gaius, was the bishop of Rome from 17 December 283 to his death in 296. Little information on Caius is available except that given by the Liber Pontificalis, which relies on a legendary account of the martyrdom of Susanna of Rome for its information. According to legend, Caius baptized the men and women who had been converted by Tiburtius and Castulus. His legend states that Caius took refuge in the catacombs of Rome and died a martyr.

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.