It is located within the facilities adjoining the cloister, some from the XII-XIII centuries, amongst which we would highlight the old chapterhouse and the old Canonical refectory, covered by a pointed barrel-vault ceiling. We would also mention the Mudejar Gothic artwork from the mid XIV century in the Treasury room, with polychromic heraldic, geometric and figurative themes. Of special note are the collections of medieval and modern religious art from Tarragona and its dioceses: altarpieces, stone and carved wood sculptures, gold-work, wrought iron work, textiles, ceramics, etc. Some of the tombstone collection is kept in the Chapel of St. Tecla the Elder.

The Diocesan Museum also holds archaeological elements dating from the first centuries of Christianity, notably including a small bronze liturgical pitcher dating to the 7th century, which was found in a tomb in the area of the cathedral. Additionally, the magnificent paleochristian Bethesda sarcophagus dating to the 5th century is built into the façade of the cathedral.