The church of San Pedro Apóstol is a 14th century Gothic temple located in the Old Quarter, between Herrería and Siervas de Jesús streets. Due to the purity of its interior architecture and its sculptural richness, it is considered one of the most beautiful Gothic temples in northern Spain.

It is currently considered an Asset of Cultural Interest, having been declared a Historic-Artistic Monument belonging to the National Artistic Treasure by decree of 3 June 1931.

Attached to the western wall, most of the building dates from the 14th century. The tower is Baroque, with a 17th-century cube and an 18th-century spire, the work of Valerio de Ascorbe, very similar to that of the tower of the nearby church of San Miguel Arcángel. Between 1892 and 1896 it underwent a restoration of which the neo-Gothic portico on the south side remains, the work of the Vitoria architect Fausto Íñiguez de Betolaza. The stained glass windows, made in Bordeaux by the Dagrant company, were installed between 1861 and 1901.

The church has a Latin cross plan, with three short naves divided into three bays, the central nave being higher than the side naves, and a transverse nave or transept of four bays plus the transept. All the bays are covered with simple ribbed arch vaulting, except for the front of the central nave, over the choir, which is vaulted with tercelets. The intersections of the ribs have decorated keystones.

The pillars have an octagonal cylindrical core with four small columns attached, or square ones. The pillars that make up the transept, transept and the third bay of the central nave are connected by diaphragm arches, seven in total, to strengthen the structure's fabric. The capitals of the pillars are finely decorated with thistles, vines, rosaceae, rows of foliage and other vegetal elements, forming a continuous band.

The Gospel (northern) nave opens onto side chapels. The nave of the Epistle (south), which is narrower, opens onto the sacristy and the Salvatierra-Adurza Chapel. The southern arm of the transept leads to the baptistery and the so-called Pórtico Viejo (Old Portico). The foot of the building is walled up because the old city wall ran along this wall.

The chevet consists of a heptagonal main apse and three pentagonal apses, one on the right side and two on the left, in which elongated windows open with pointed arches and baquetons in archivolts and jambs. They have fine mullions and tympanums pierced by three-lobed openings in the main chapel and by tangent flowered oculi in the side chapels. In the apse of the main chapel there is also another series of windows superimposed on the previous ones: they are smaller and also have mullions and tympana opened by three quadrifoliated rosettes. The slenderness and elegance of this chancel are reminiscent of the architecture of the churches of the Dominican and Franciscan mendicant orders.