Sitting in a wide valley opening onto the Cantabrian Sea, Mondoñedo is one of the most evocative capitals of the old Kingdom of Galicia. Its old town has been declared Property of Cultural Interest.

The Cathedral of San Martiño, a National Monument, preserves the original Romanesque main front with a beautiful rose window and baroque towers. Inside, there is also a juxtaposition of styles, an ogival dome held up by pointed arches, mural paintings from the 15th and 16th centuries, and a baroque high reredos. It also has a stone reredos, the only one in the Romanesque style. It houses the Diocesan Cathedral Museum, one of the most important museums of religious art in Spain.

In the actual cathedral square you can find the Episcopal Palace; the Old Consistory, which dates from the 16th century and is currently the local library; the monument to the writer Álvaro Cunqueiro; and several houses with arcades. Near the square is Charles V's coat of arms engraved on the Fonte Vella (old fountain). The Santa Catalina Seminary, an 18th-century monumental building, can also be found behind the cathedral. The convents of Concepción and San Pedro de Alcántara are Baroque. Another interesting area is the Os Muiños neighbourhood, because of its traditional architecture and structure made up of canals.

Two kilometres away from Mondoñedo are the monastery of San Martín and, at the top of the hill, a small hermitage from where it is said that, around the 9th century, the bishop San Gonzalo sank the Norman fleet by reciting Hail Marys.