Colusa is one of the 58 counties in the state of California. It was created in 1850, making it one of the original counties of the state. As of the 2010 census the population was 21,419, up from 18,804 in 2000. The county seat and largest city is Colusa.
The county covers 1,156.22 square miles (2,994.6 km2) of which 1,150.68 square miles (2,980.2 km2) is land. It is bordered by Glenn County to the north, Butte County to the northeast, Sutter County to the east, Yolo County to the south and Lake County to the west.