Santo Tomas, officially the Municipality of Santo Tomas (Ibanag: Ili na Santo Tomás; Ilocano: Ili ti Santo Tomás; Tagalog: Bayan ng Santo Tomás), is a municipality in the province of Isabela, Philippines. According to the 2024 census, it has a population of 25,997 people.

The municipality was established in 1949 from several barrios of Cabagan.

In 1952, Santo Tomas lost two 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.

Santo Tomas is a landlocked municipality situated in the northern portion of the province of Isabela. It is bounded to the west by Quezon, to the south by Delfin Albano, to the southeast by Tumauini, to the north and northeast by Cabagan and the Cagayan River.

Santo Tomas is situated 45.06 kilometres (28.00 mi) from the provincial capital Ilagan, and 473.98 kilometres (294.52 mi) from the country's capital city of Manila.

Santo Tomas is politically subdivided into 27 barangays.  Each barangay consists of puroks while some have sitios.

Uauang-Galicia is now the population is 0 according in 2010 Census.

In the 2024 census, the population of Santo Tomas was 25,997 people, with a density of 400 inhabitants per square kilometre or 1,000 inhabitants per square mile.