According to popular belief, St. Petronilla was St. Peter's child. She was referred to be his physical daughter until the XVII Century, and ever since then, it has been believed that she is actually his spiritual daughter. According to legends found in Manichcan archives, St. Peter cured her of her palsy.

According to legends recounted in The Golden Legend and St. Marcellus' writings, Peter, who believed his daughter was too beautiful, asked God to give her a fever so that he wouldn't have to treat her until she started to grow in her understanding of God's love. It is also alleged that she turned down Count Flaccus' proposal of marriage. Tradition holds that she passed away naturally, but there are other reports of her being martyred.

Additionally, Petronilla was said to be related to St. Domitilla, who was banished to Pandateria in the first century, and whose land on Via Ardentina was turned into a catacomb cemetery. Petronilla is referred to as a martyr in the inscriptions there.

On the location of her tomb, a basilica was constructed during the pontificate of Siricius (384–399). Her relics were moved to St. Peter's Cathedral, where a chapel was built in her honor, and Gregory III created a place of public worship there in the eighth century. Both Charlemagne (d. 814) and Carlomen (d. 771) were regarded as St. Peter's adoptive sons, and they as well as the French kings who followed them thought of Petronilla as their sister. Her chapel was adopted by the French kings as their chapel. Her emblem, like St. Peter's, is a set of keys.

St. Petronilla is the patron saint of the dauphins of France, mountain travelers, treaties between Popes and Frankish emperors and fever. Her feast day is celebrated on May 31.

Other Saints of the Day
Saint Vitalis
Saint Crescentian
Saint Mechtildis
Saint Paschasius
Saint Thomas Du