Petronila of Aragon (1135- October 17 1174, Barcelona) was the daughter of King Ramiro II of Aragon and Agnes of Poitiers , a.k.a. Agnes of Aquitaine. (Petronila is also referred to by various sources as "Petronilla", "Petronilla Ramírez", "Pétronille", or "Petronella").

In 1134, when his brother Alfonso I of Aragon died heirless, Ramiro was bishop of Barbastro-Roda. He was given papal dispensation to abdicate his monastic vows in order to secure the succession to the throne. King Ramiro the Monk, as he is known, married Agnes of Poitiers, produced an heiress, Petronila, bethrothed her (aged two) to Ramon Berenguer IV of Catalonia, abdicated in her favour and returned to monastic life.

Petronila married Ramon Berenguer in 1150. While he was alive, they nominally ruled their possessions separately, although the count had the final say over both Aragon and Catalonia. Upon his death Petronila renounced the crown of Aragon in favour of her eldest son Ramon, who, in compliment to the Aragonese, changed his name to Alfonso. Alfonso II of Aragon, I of Barcelona was the first ruler of both Aragon and Catalonia. This effected the dynastic union between the two countries that was to last until their incorporation in the kingdom of Spain in 1707.