An early Roman martyr, the date and details of whose death are unknown.
She belonged to the family of Domitilla, in whose catacomb she is described as a martyr. Her fictitious 6th-century Acts make her a daughter of Peter, who refused marriage to a Count Flaccus; he threatened to have her killed, but she died after three days' fasting. Her sarcophagus was moved to St Peter's in the 8th century and her chapel there became that of the kings of France. Charlemagne and Carloman were considered St Peter's adopted sons, and his supposed daughter became their patroness. The chapel was embellished by kings of France and by later popes who employed Michelangelo and Bramante. Mass on her feast day, 31 May, is offered for France and attended by French residents. St Petronilla occurs fairly frequently in English late medieval stained glass and painted screens, and her usual emblem is a set of keys, presumably borrowed from St Peter.
AA.SS. Maii VII (1688), 420–2; C.M.H., pp. 280, 285; H. Delehaye, Sanctus (1927), pp. 118–20.