San Agustin, officially the Municipality of San Agustin, is a municipality in the province of Isabela, Philippines. According to the 2020 census, it …San Agustin, officially the Municipality of San Agustin, is a municipality in the province of Isabela, Philippines. According to the 2020 census, it has a population of 22,096 people. The town derived its name from Saint Augustine of Hippo.