The church of San Juan, also known as the church of San Nicolás, is located in Praza do Camiño, an important stop on the Camino de Santiago. In the 12th century, the Order of the Knights of St. John of Jerusalem was established here to run a pilgrims' hospital.

Built in the 13th century, its construction was carried out by one of the main collaborators of Master Mateo, known for his work on the Cathedral of Santiago. The church was originally located next to the Miño River, in a strategic place on the French Way, but was moved in the 1950s to its current location due to the construction of the Belesar reservoir. The numbered ashlars can still be seen as a testimony of its reconstruction.

The temple has the appearance of a church-fortress, with a single nave and a semicircular chancel. The main facade, which imitates a defensive structure, has battlements, arrow slits and towers on both sides. It has a large arch of discharge, divided by a tornalluvias, and an imposing rose window, one of the largest in Galicia. The main portico, with three decorated archivolts, is inspired by the workshop of master Mateo. The inner archivolt shows the twenty-four Elders of the Apocalypse playing instruments, while the plain tympanum houses the figure of the Pantocrator. The capitals of the columns are decorated with vegetal motifs.

The north and south side doors are also preserved. The tympanum of the north door shows the Annunciation, while the south door depicts a figure wearing a mitre and chasuble. Both doors have archivolts with rich ornamentation.

The interior of the church is sober and elegant, with a nave covered by a pointed barrel vault, and the chancel with barrel and shell vaults. A rose window in the triumphal arch illuminates the nave. On each side, two baldachins flank the entrance.

Two spiral staircases, located on both sides of the main door, lead to a corridor under the rose window and to a crenellated walkway that surrounds the four towers.

The church of San Juan de Portomarín is considered one of the best examples of Galician Romanesque architecture and was declared a Historic-Artistic Monument in 1931.