Barangay Old San Mariano

Barangay Old San Mariano is a mid-sized barangay in the municipality of San Mariano, located in the province of Isabela, in the Cagayan Valley region of the Philippines.

According to the 2020 Philippine census, Barangay Old San Mariano had a population of 2,274. This made it the 8th largest of 36 barangays in San Mariano by population, placing it in the upper half of the municipality. Residents of Barangay Old San Mariano accounted for approximately 3.78% of the total population of San Mariano, which stood at 60,124 in the same census year.

Between the 2015 and 2020 censuses, the population of Barangay Old San Mariano changed from 2,164 to 2,274, a shift of +5.1% over the five-year period. This places the barangay among those with growing populations in San Mariano. Barangay Old San Mariano is officially classified as rural, a designation applied to barangays with lower population density and predominantly non-built-up land use.

San Mariano is a municipality comprising 36 barangays, and serves as the governing unit directly responsible for local services in Barangay Old San Mariano, 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 Old San Mariano ranks 209th 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.