Lassen County is a county located in the northeastern portion of the U.S. state of California. As of 2000 the population was 33,828. The county seat is Susanville, the only incorporated city in the county.

Lassen County was formed on April 1, 1864 from parts of Plumas and Shasta Counties following the two-day conflict known as the "Sagebrush War", also called the Roop County War, that started on February 15, 1863. Due to uncertainties over the California border, what is now Lassen County was part of the unofficial Nataqua Territory and Roop County, Nevada during the late 1850s and early 1860s. The county was named after Peter Lassen, along with Lassen Peak, which is in adjoining Shasta County. Lassen was one of General John C. Fremont's guides, etc.