Bermillo de Sayago is a municipality in the province of Zamora, within the autonomous community of Castile and León, Spain. It lies in the comarca of Sayago, in the western part of the province near the border with Portugal.
The town functions as the territorial capital of the Sayago comarca, although this role carries no official status. Its central position within the district and its notably larger population relative to surrounding settlements are the reasons it has assumed this function. According to the 2009 census (INE), the municipality had a population of 1,259 inhabitants.
Bermillo de Sayago is also the headquarters of Sayagua, a mancomunidad, or association of municipalities, that provides services across the Sayago comarca. These services, which include water supply and recycling, are of a kind that the area's much smaller towns could not afford individually.