Colusa County is a county located in the Central Valley of the U.S. state of California, northwest of state capital Sacramento. As of 2000, its population was 18,804. The county seat is Colusa. Colusa County is one of the original counties of California, created in 1850 at the time of statehood.

Parts of the county's territory were given to Tehama County in 1856 and to Glenn County in 1891. The county was named after the 1844 Rancho Colus Mexican land grant to John Bidwell. The name of the county in the original state legislative act of 1850 was spelled Colusi, and often in newspapers was spelled Coluse. The word is derived from the name of a Native American tribe living on the west side of the Sacramento River. The name was changed to Colusa shortly after . This however is disputed. The actual origin of the name is unknown.