San Mateo (pronounced locally as /sæn mə-tay-oh/ not /sænˈmäCHō/ Spanish: St. Matthew) is a 1st municipal income class municipality in the province of Isabela, Philippines. According to the 2015 census, it has a population of 64,505 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".

According to the 2016 Cities and Municipalities Competitiveness Index conducted by the National Competitiveness Council, San Mateo took the 86th spot overall and ranked 99 among the first class and second class municipalities in the Philippines.

First named as "Marasat", it was a former barangay of Cauayan, then Santiago. It was called the Municipality of Yoshisawa under the government of the Japanese Imperial Army during the Second World War. After the liberation, it was finally created and named San Mateo by virtue of Presidential Executive Order No. 97 on March 17, 1946 by then President Sergio Osmeña. It was proclaimed as the Agro-Ecological Destination in Cagayan Valley by President Gloria Macapagal Arroyo by virtue of Presidential Proclamation No. 1685 dated December 1, 2008.

San Mateo is situated in the Southwestern part of Isabela. It is bounded on the north by Aurora, on the northeast by Cabatuan, on the east by Cauayan City, on the east-southeast by Alicia, on the south by Ramon and on the west Alfonso Lista in the Province of Ifugao.

San Mateo is 10 kilometres (6.2 mi) from Cabatuan, 8 kilometres (5.0 mi) from Ramon along the national highway, and 17 kilometres (11 mi) from Alicia along the provincial road. It is 25 kilometres (16 mi) away from the Santiago City and 24 kilometres (15 mi) away from the Cauayan City. It is 54 kilometres (34 mi) south of Ilagan, capital city of Isabela on the national highway and approximately 6½ hours trip to Manila with a distance of 350 kilometres (220 mi). It has a total area of approximately 12,059.83 hectares.

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