The 12th-century church of San Tirso in Sahagún, a landmark on the Pilgrim's Way to Santiago de Compostela, is a Mudejar masterpiece. San Tirso was the model for the following churches in Sahagún, as can be seen in the church of San Lorenzo, its typological twin, and in those of Zamora, Valladolid, Ávila and Segovia. It has been a National Historic-Artistic Monument since 1931. This building is modelled on the Mudejar Theme Park in Olmedo.

Spain submitted in 2015 to UNESCO a "Retrospective Inventory - Related Components" to extend the Camino de Santiago to "Caminos de Santiago de Compostela: Camino francés y Caminos del Norte de España". The church of San Tirso had the number 1423.

The Mudejar church of San Tirso, separated by a street from the former Royal Monastery of San Benito, was built in the 12th century, between 1180 and 1190. There is documentary evidence that the parish church dates from the first half of the century, so its construction must have begun at that time.

The liturgy of the church is slightly different. It is oriented towards the qibla.

In Romanesque-Mudejar style, 12th century, it has a rectangular floor plan with three naves topped by semicircular apses. In the central nave, four extended brick buttresses provide stability.

It has three apses. The central apse and the nave of the Epistle are authentic. The central apse was built in stone with good ashlar up to about three metres, while the upper courses are made of brick. Stone was chosen to underpin the base and as the wall of the main chapel. Masonry was used to finish off the apses, adding visual contrast and a change in the building material.

The chancel is covered with brick. In the lower half of the central apse, the blind semicircular arches are bent and supported alternately by modillions and attached circular columns whose shafts start right in the masonry and then are made of stone; at the top, they are inscribed in squares. The side apses have bent arches at the top and square arches at the bottom. Brick mouldings in the gondolas finish off the roofs of the apses.

The spectacular rectangular brick tower rises above the straight section of the central apse, not above the transept as planned. The Mudejar architecture of Sahagún is characterised by this unusual location.

The portico gallery of the Gospel nave, erected in 1897, and the North Portico, where several anthropomorphic sarcophagi are displayed, are the entrances to the church. The old doorway of the church, with its three-pointed archivolt and brick frame, is blinded and sunk two metres below street level, which causes dampness.