San Mariano is a first class municipality in the province of Isabela, Philippines. According to the 2010 census, it has a population of 51,438 people.San Mariano has a total land area of 146,950 hectares (363,100 acres). It constitutes 13.78 percent of the total land area of the Province, and is the province's largest and the country's third largest municipality.The municipality lies in the eastern part of the Province of Isabela.