Caso ( Asturian: Casu) is a municipality in the Spanish Principality of Asturias. It shares a boundary to the North with Piloña; to the East with Ponga; to the South with León and to the West with Sobrescobio and Laviana.
Most of the inhabitants of Caso live in the Nalón Valley, on which the largest town is the capital El Campu. The town is divided in two neighbourhoods: L'Arrobiu and El Barru.
The municipalities of Caso and Sobrescobio together constitute the Redes Natural Park, designated by the UNESCO as a Biosphere Reserve. The Natural Monument of the Cueva Deboyu (the Nalón River passes through a cave in the mountain) is also located in Caso. Caso is famous for its landscape, European beech forests, wood handcraft and its Casín cheese.