The Cathedral of San Pedro, an authentic jewel of Castilian Romanesque architecture in the city of Soria.

Since 1959 it has accumulated the title of co-cathedral, date from which it shares the cathedral seat with El Burgo de Osma; until that year it was called the collegiate church of San Pedro.

The primitive church could have its origin in the years when Alfonso I the Battler, occupied in Castilian affairs due to his marriage to Urraca de Castilla, undertook the repopulation of Soria (1109-1114).

There has been no archaeological record of the old church but it is documentary, described by historiography as "simple and rustic church, [...] reduced parish".

On the outer walls of the north side of the cloister there is a window with an appearance and arrangement that reminds us of Asturian double and triple openings.

There are three semicircular arches with soged archivolts, followed by others with diamond points, lowered onto two central columns and onto the jambs of the gap.

The capitals of the two columns represent rough leaves finished in thick caulicles like balls. Furthermore, underneath there is a very mutilated window arch, which according to Gaya Nuño could have been a horseshoe, and sunk into the ground what looks like a semi-circular doorway with buried supports.

Two small Romanesque loopholes also stand out on the north wall.

The orientation, configuration of the walls and preserved remains suggest an integration of the first Romanesque temple in the cloister of the monastic Romanesque temple.

The Marrón texts and according to an inscription that appears in the Azogue chapel, place the dating of this first church to more than 800 years after its reconstruction in 1573, that is, around 770, so it should be Mozarabic.

The church was donated by the council of Soria to the bishop of Osma, Don Juan, on July 26, 1148.

In 1152 the bishop of Osma Don Juan donated the church to the canons of the rule of San Agustín and elevated the church to the category of collegiate church.

Once the canons were constituted as a monastic community, they decided to tear down the old temple and erect a new one. For this they counted on the favor of the Castilian monarchs, which was reflected in numerous donations and privileges.