The earliest record of this monastery dates from 942 and its foundation is attributed to Saint Fructuosus of the Benedictine Order. Emperor Charles V granted it the privilege of housing a School of Theology between 1648 and 1556, where Father Feijóo taught. The ecclesiastical confiscations of 1835 forced the Order of Saint Benedict to abandon Poio, and the Mercedarians arrived at the monastery in 1890. The building has two cloisters: the rib-vaulted Cloister of the Processions, built in the 16th century in the Renaissance style, which has a Baroque fountain in the garden; and the Baroque Cloister of the Orange Trees, covered by a 200-square-metre mosaic depicting the Camino de Santiago (‘Way of St. James’) from France, which was built between 1747 and 1749. The church combines elements of Renaissance and Baroque styles. The high altar was built in 1735 and has statues of Mercedarians, the Virgin of Mercy and Saint John the Baptist, patron saint of the church. The altarpiece in the Chapel of Christ is situated on the former sacristy, where the Suebi-Visigoth tomb of Saint Trahamunda is located. Additionally, Galicia's largest hórreo (a traditional raised granary) is located in this monastery’s garden.