The first known human presence in the territory of San Martín goes back to the neolithic period, as witnessed by tumulus remains found in Os Pedrousos (Teixera).

There are remains of ancient fortified settlements at San Isidro and Pico de la Mina, near to Bousoño, both situated in elevated positions clearly motivated by defensive considerations.

In Tabladas and on the banks of the river Santalla traces of Roman smelting works have been found, including stone crucibles and slag heaps.

During the medieval period San Martín, together with neighbouring Santa Eulalia and the rest of the Castropol district, was granted to the church in Oviedo by King Alfonso VII in 1154.

With the authorisation of Pope Benedict XIII, the bishopric's lands were being sold to the local inhabitants as independent municipalities, to pay for the successive wars in which Spain had been involved.