It has been historically linked to the Ancient Province of Mondoñedo and also linked to the arrival of Briton immigrants during the Dark Ages (5th and 6th centuries) fleeing by sea from the British Isles (see Bishop Maeloc, Britonia and San Martiño de Mondoñedo).

The foundation of Foz dates back to pre-Roman times, as attested by the existing forts Fazouro and Pena do Altar.

Its foundation may originate in the time of the Ártabros or, according to the historian Amor Meilán, a factory may have been established by the Tartessos.

During the ninth century the town had flourished since the establishment of the episcopal headquarters in San Martiño de Mondoñedo.

At the time of Ferdinand and Isabella, Foz kept certain privileges and exemptions as a result of its commercial importance.