San Guillermo (Spanish for Saint William), officially the Municipality of San Guillermo, is a municipality in the province of Isabela, Philippines. According to the , it has a population of people.
The town was named after Eugenio Guillermo, former mayor of Angadanan.
On June 17, 1967, the municipality of San Guillermo was created by virtue of Republic Act No. 4906, sponsored by Melanio Singson, the then-representative of Isabela's lone district. It used to consist of 15 barangays from Angadanan (i.e. Anonang, Colorado, Calaoagan, Dipacamo, Peredo Edcor, Guam, Nakar, Palawan, Progreso, San Francisco Norte, San Mariano Norte, San Mariano Sur, Villa Rose, Villa Sanchez, and Villa Teresita) and four from Echague (i.e. Aringay, Dingading, San Francisco Sur and Sinalugan).
Due to a noticeable increase in the rate of population migration and potential economic growth, additional barangays were created during the term of Mayor Alvaro Filart, such as barangays Burgos, Dietban, Estrella, Rizal, San Rafael and Villa Remedios. Subsequently, barangay Peredo Edcor was split into two barangays, namely: Centro 1 and Centro 2, which is now the poblacion and the municipality's official seat.
San Guillermo is situated from the provincial capital Ilagan, and from the country's capital city of Manila.
San Guillermo is politically subdivided into barangays. Each barangay consists of puroks while some have sitios.
In the 2024 census, the population of San Guillermo was 21,043 people, with a density of .
As a municipality in the Province of Isabela, government officials at the provincial and municipal levels are voted by the town. The provincial government has political jurisdiction over most local transactions of the municipal government.
The Municipality of San Guillermo is governed by a mayor, designated as its local chief executive, and by a municipal council as its legislative body in accordance with the Local Government Code. The mayor, vice mayor, and the municipal councilors are elected directly through elections held every three years.
Barangays are also headed by elected officials: Barangay Captain, Barangay Council, whose members are called Barangay Councilors. The barangays have SK federation which represents the barangay, headed by SK chairperson and whose members are called SK councilors. All officials are also elected every three years.
San Guillermo, belonging to the sixth legislative district of the province of Isabela, currently represented by Hon. Faustino A. Dy V.
The Schools Division of Isabela governs the town's public education system. The division office is a field office of the DepEd in Cagayan Valley region. The San Guillermo Schools District Office oversees all public and private elementary and high schools within the municipality.