St. Sixtus I, (born, Rome [Italy]?—died c. 125, Rome?; feast day April 3), pope from approximately 115 to about 125. He succeeded St. Alexander I and ruled the church under the Roman emperor Hadrian. Although authoritative sources vary on the dates of his pontificate, they all agree that he reigned for 9 or 10 years. Sixtus’s martyrdom is unproved. He was succeeded by St. Telesphorus.