Petronilla was a Roman virgin, probably martyred at the end of the first century. Her grave in the catacomb of Domitilla on the Via Ardeatina, beside Sts Nereus and Achilleus, is named in almost all the old lists of the martyrs' tombs, and excavations have confirmed it. A painting made shortly after 356 shows her welcoming a dead woman into heaven and is labelled 'Petronilla Martyr'. The source treats this as certain historical evidence that Rome honoured her as a martyr in the fourth century.

Her sarcophagus bore the name Aurelia Petronilla. This suggests she was related to the Christian Flavii and explains her burial in the catacomb of the Flavian Domitilla. She may have suffered under Domitian, or perhaps later.

A legend going back at least to the sixth century made her a daughter of St Peter, probably because of her name and her ancient tomb. It presents her as a virgin rather than a martyr. In 757 her remains were moved to a chapel beside St Peter's, and she became the patron of the treaties between the popes and the Frankish kings. Her relics now rest at an altar in St Peter's.

Thin entry · the 1897 book gives only 104 words. We have not filled the gap with invented detail.

A certain officer or “Count33 Flaccus having greatly admired her beauty, sent soldiers to her, to ask her to be his wife. She replied sharply, “If he wants me to marry him, let him not send rough soldiers to woo me, but respectable matrons, and give me time to make up my mind.33 Whereupon the soldiers withdrew abashed. But before Flaccus had obtained matrons to convey his offer, Petronilla was dead.

Virgin, probably martyred at Rome at the end of the first century.

Almost all the sixth- and seventh-century lists of the tombs of the most highly venerated Roman martyrs mention St. Petronilla's grave as situated in the Via Ardeatina near Sts. Nereus and Achilleus (De Rossi, "Roma sotterranea", I, 180-1). These notices have been completely confirmed by the excavations in the Catacomb of Domitilla. One topography of the graves of the Roman martyrs, "Epitome libri de locis sanctorum martyrum", locates on the Via Ardeatina a church of St. Petronilla, in which Sts. Nereus and Achilleus, as well as Petronilla, were buried (De Rossi, loc. cit., 180). This church, built into the above-mentioned catacomb, has been discovered, and the memorials found in it removed all doubt that the tombs of the three saints were once venerated there (De Rossi in "Bullettino di archeol. crist.", 1874 sq., 5 sqq.). A painting, in which Petronilla is represented as receiving a deceased person (named Veneranda) into heaven, was discovered on the closing stone of a tomb in an underground crypt behind the apse of the basilica (Wilpert, "Die Malereien der Katakomben Roms", Freiburg, 1903, plate 213; De Rossi, ibid., 1875, 5 sqq.). Beside the saint's picture is her name: Petronilla Mart. (yr). That the painting was done shortly after 356, is proved by an inscription found in the tomb. It is thus clearly established that Petronilla was venerated at Rome as a martyr in the fourth century, and the testimony must be accepted as certainly historical, notwithstanding the later legend which recognizes her only as a virgin (see below). Another known, but unfortunately no longer extant, memorial was the marble sacrophagus which contained her remains, under Paul I translated to St. Peter's. In the account of this in the "Liber Pontificalis" (ed. Duchesne, I, 466) the inscription carved on the sacrophagus is given thus: Aureae Petronillae Filiae Dulcissimae (of the golden Petronilla, the sweetest daughter). We learn, however, from extant sixteenth-century notices concerning this sacrophagus that the first word was Aur. (Aureliae), so that the martyr's name was Aurelia Petronilla. The second name comes from Petro or Petronius, and, as the name of the great-grandfather of the Christian consul, Flavius Clemens, was Titus Flavius Petronius, it is very possible that Petronilla was a relative of the Christian Flavii, who were descended from the senatorial family of the Aurelii. This theory would also explain why Petronilla was buried in the catacomb of the Flavian Domitilla. Like the latter, Petronilla may have suffered during the persecution of Domitian [Roman emperor 81–96], perhaps not till later.

In the fourth-century Roman catalogue of martyrs' feasts, which is used in the "Martyrologium Hieronymianum", her name seems not to have been inserted. It occurs in the latter martyrology (De Rossi-Duschesne, "Martyrol. Hieronym.", 69), but only as a later addition. Her name is given under 31 May and the Martyrologies of Bede [English monk and historian (d. 735)] and his imitators adopt the same date (Quentin, "Les martyrologes historiques", Paris, 1908, 51, 363, etc.). The absence of her name from the fourth-century Roman calendar of feasts suggests that Petronilla died at the end of the first or during the second century, since no special feasts for martyrs were celebrated during this period. After the erection of the basilica over her remains and those of Sts. Nereus and Achilleus in the fourth century, her cult extended widely and her name was therefore admitted later into the martyrology. A legend, the existence of which in the sixth century is proved by its presence in the list of the tombs of the Roman martyrs prepared by Abbot John at the end of this century (De Rossi, "Roma sotterranea", I, 180), regards Petronilla as a real daughter of St. Peter. In the Gnostic apocryphal Acts of St. Peter, dating from the second century, a daughter of St. Peter is mentioned, although her name is not given (Schmid, "Ein vorirenöische gnostisches Originalwerk in koptischer Sprache" in "Sitzungsber. der Berliner Akademie", 1896, 839 sqq.; Lipsius, "Die apokryphen Apostelgeschichten u. Apostellegenden", II, i, Brunswick, 1887, 203 sqq.). The legend being widely propagated by these apocryphal Acts, Petronilla was identified at Rome with this supposed daughter of St. Peter, probably because of her name and the great antiquity of her tomb. As such, but now as a virgin, not as a martyr, she appears in the legendary Acts of the martyrs St. Nereus and Achilleus and in the "Liber Pontificalis" (loc. cit.). From this legend of Sts. Nereus and Achilleus a similar notice was admitted into the historical martyrologies of the Middle Ages and thence into the modern Roman Martyrology. In 757 the coffin containing the mortal remains of the saint was transferred to an old circular building (an imperial mausoleum dating from the end of the fourth century) near St. Peter's. This building was altered and became the Chapel of St. Petronilla (De Rossi, "Inscriptiones christianae urbis Romae", II, 225). The saint subsequently appears as the special patroness of the treaties concluded between the popes and the Frankish emperors. At the rebuilding of St. Peter's in the sixteenth century, St. Petronilla's remains were translated to an altar (still dedicated to her) in the upper end of the right side-aisle (near the cupola). Her feast falls on 31 May.