Barangay San Mariano Sur

Barangay San Mariano Sur forms part of San Guillermo, a municipality in Isabela, and is classified as a small barangay.

According to the 2020 Census of Population and Housing conducted by the Philippine Statistics Authority, Barangay San Mariano Sur had a population of 608. This made it the 13th largest of 26 barangays in San Guillermo by population, placing it in the middle tier of the municipality. Residents of Barangay San Mariano Sur accounted for approximately 2.91% of the total population of San Guillermo, which stood at 20,915 in the same census year.

Between the 2015 and 2020 censuses, the population of Barangay San Mariano Sur changed from 644 to 608, a shift of -5.6% over the five-year period. This places the barangay among those with contracting populations in San Guillermo. The Philippine Statistics Authority classifies Barangay San Mariano Sur as rural, a designation applied to barangays with lower population density and predominantly non-built-up land use.

San Guillermo is a municipality comprising 26 barangays, and serves as the governing unit directly responsible for local services in Barangay San Mariano Sur, including public health, sanitation, peace and order, and civil registration. Within the wider province of Isabela, which contains 1,055 barangays across its component cities and municipalities, Barangay San Mariano Sur ranks 878th by 2020 population. Isabela is classified as a 1st by the Bureau of Local Government Finance, a category that reflects the province's annual regular income.

The barangay's legislative and executive affairs are handled by a Punong Barangay and the Sangguniang Barangay, elected nationwide during the Barangay and SK Elections. The next BSKE is scheduled for Monday, November 2, 2026, following the synchronized election schedule set by COMELEC Resolution No. 11191.