Modoc is one of the 58 counties in the state of California. It was created in 1874 from the eastern part of Siskiyou County. As of the 2010 census the population was 9,686, up slightly from 9,449 in 2000. The county seat and largest city is Alturas.
The county covers 4,203.37 square miles (10,886.7 km2), of which 3,944.10 square miles (10,215.2 km2) is land. It is bordered by Lassen County on the south, Shasta County on the southwest, Siskiyou County on the west, Klamath County, Oregon on the northwest, Lake County, Oregon on the northeast, and Washoe County, Nevada on the east.
Modoc is one of the counties wishing to secede from California, and form the new state of Jefferson.