The monastery of San Pedro el Viejo, formerly called "Iglesia antigua de San Pedro el Viejo de Huesca", is a 12th century Romanesque building located in the old quarter of the city of Huesca. Listed as a National Monument since 1885, its architecture and sculpture make it one of the most important historical sites of Aragonese Romanesque architecture.

What was once its chapterhouse serves as a royal pantheon, with the tombs of two kings of Aragon: Alfonso I the Battler and his brother and successor, Ramiro II the Monk.

Archaeological evidence from this site suggests the existence of a Roman temple or necropolis. Later, a Visigothic church was built in which, after the Muslim conquest of the city in 719, the Mozarabic population was able to practise Christian worship under the patronage of Saint Peter.

When Huesca was reconquered in 1096, the Aragonese monarch Pedro I ceded "eclesiam Santi Petri illam antiquam" - the ancient church of Saint Peter - to the then prosperous Benedictine monastery of Saint-Pons-de-Thomières located in Languedoc, which in 1117 began the construction of the church, cloister and other monastic buildings.

The church consists of three barrel-vaulted naves and their respective semicircular apses with a half-sphere vault, built in the first half of the 12th century in the Romanesque style. The dome was built around 1240 in the Gothic style.

At the foot of the central nave, on the north wall, there are Gothic mural paintings (13th century) with the themes of David and his battle with Goliath and Moses and the burning bush.

The main altarpiece is made of polychrome wood and is presided over by the titular figure of the church, Saint Peter, with the triple tiara of the papacy. It is dated 1602 and is the work of the Navarrese sculptors Juan de Berrueta4 and Juan de Alli5 and Juan de Berrueta5 , who lived in Sangüesa.

At the foot of the central nave is the choir; the choir stalls are the work of Juan Bierto and are dated 1506; he is also the author of the choir stalls in the monastery of Casbas.