Sierraville is a census-designated place in Sierra County, California, United States. It is at the southern end of the huge Sierra Valley which is used primarily as pasture and is also at the junction of California State Route 49 and California State Route 89 southwest of Loyalton. Sierraville has a post office with ZIP code 96126. The post office opened under the name Sierra Valley in 1862 and became known as the Sierraville post office in 1899. The population was 200 as of the 2010 census.