In 1913 the building was declared historical monument, and bought by the Town Hall in 1928 to be inaugurated in 1932 as a museum. This collection had formerly been housed in a building between Garibay and Andia Streets, and later in another in Urdaneta Street in 1909. The museum has a ground floor and two storeys. The ground floor comprises of the church and lower cloister, with the Archeology Section, exhibition halls and entrances. Finally, the second storey is dedicated in its entirety to contemporary Basque painting.
Sculptures are also to be found in several parts of the museum.
Architecturally, the two most important parts of the museum are the cloister and the church. In the latter, XVIth century architecture and the XXth century Sert canvases combine harmoniously. The cloister houses the archaeology exhibits. Alongside engraved tombstones, baptismal fonts, stone fragments of hermitages and churches, a medieval cross and coats of arms, there is an interesting collection of discoid steles. This is truly one of the best collections of steles to be found in the world.