Thermalito is a census-designated place in Butte County, California, United States. The population was 6,646 at the 2010 census, up from 6,045 at the 2000 census. The area is generally considered an integral part of Oroville, and borders the town on two sides, in addition to the near-enclave of the city of Oroville surrounded by Thermalito on all sides, with the exception of the extremely narrow spit of land connecting Afterbay to Oroville proper.