Santo Tomas is a fourth class municipality in the northern part of province of Isabela, Philippines. According to the 2010 census, it had a population of 21,688 people.In 1952, Santo Tomas lost 2 barrios when the barrios of Abut and Minagbag were transferred to the newly created town of Mallig. In 1961, those barrios were transferred to the newly created municipality of Quezon