Barangay San Agustin

Classified as a mid-sized barangay, Barangay San Agustin is one of the administrative divisions of Castillejos in Zambales, Central Luzon, Philippines.

According to the 2020 Census of Population and Housing conducted by the Philippine Statistics Authority, Barangay San Agustin had a population of 4,827. This made it the 5th largest of 14 barangays in Castillejos by population, placing it in the upper half of the municipality. Residents of Barangay San Agustin accounted for approximately 7.11% of the total population of Castillejos, which stood at 67,889 in the same census year.

The barangay recorded 4,545 residents in the 2015 intercensal count and 4,827 residents in the 2020 Census — a net change of +6.2%, consistent with a growing settlement. The Philippine Statistics Authority classifies Barangay San Agustin as rural, a designation applied to barangays with lower population density and predominantly non-built-up land use.

Castillejos is a municipality comprising 14 barangays, and serves as the governing unit directly responsible for local services in Barangay San Agustin, including public health, sanitation, peace and order, and civil registration. Within the wider province of Zambales, which contains 247 barangays across its component cities and municipalities, Barangay San Agustin ranks 30th by 2020 population. Zambales is classified as a 2nd 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.