Colusa County is a county located in the U.S. state of California. As of the 2020 census, the population was 21,839. The county seat is Colusa. It is in the northern Sacramento Valley, northwest of the state capital, Sacramento.

Colusa is a city and county seat of Colusa County, California, located in the Sacramento Valley region of the Central Valley. The population was 5,971 at the 2010 census, up from 5,402 at the 2000 census. Colusi originates from the local Coru Native American tribe, who in the 1840s lived on the opposite side of the Sacramento River.