San Mateo, officially the Municipality of San Mateo, is a 1st class municipality in the province of Isabela, Philippines. According to the 2020 census, it has a population of 66,663 people. San Mateo is one of the cleanest town in the region. The administrators encourage the people of the municipality to use paper bags instead of plastic bags in line with its claim as an "Agro-Ecological Town".