San Martín del Rey Aurelio is a municipality of the Autonomous Community of the Principality of Asturias in northern Spain. There are three main towns in the municipality: Sotrondio, El Entrego, and Blimea, and many villages, such as Santa Bárbara and La Hueria. It is situated in the central part of Asturias, just below the Sierra de San Mamés, part of the Cantabrian Mountains. The Nalón River, the longest river in Asturias, crosses the municipality. The Redes Natural Park is located nearby.