San Carlos is a census-designated place (CDP) in Gila County, Arizona, United States. The population was 3,716 at the 2000 census. San Carlos is the largest community in and the seat of government for the San Carlos Apache Indian Reservation. San Carlos' economy is underdeveloped and is based mainly on retail service industries, construction trades, and public administration. With 58.8% of the population under the poverty level and a median household income of $13,412, San Carlos remains one of the poorest Native American communities in the United States.