Lassen County (/ˈlæsən/ ^ⓘ LASS-ən) is a county located in the northeastern portion of the U.S. state of California. As of the 2020 census, the population was 32,730, and was estimated to be 28,340 in 2024, Its county seat and the largest city is Susanville.

Lassen County comprises the Susanville, California micropolitan statistical area. A former farming, mining, and lumber area, its economy now depends on employment at one federal and two state prisons; the former in Herlong and the latter two in Susanville. In 2007, half the adults in Susanville worked in one of the facilities.

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 Sunday morning, February 15, 1863. Due to uncertainties over the California border, the area that 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 by California after Peter Lassen, along with Lassen Peak, which is in adjoining Shasta County. Lassen was one of General John C. Fremont's guides, and a famous trapper, frontiersman, and Indian fighter. He was murdered under mysterious circumstances near the Black Rock Desert in 1859, and his murder was never solved.

By the 1880s small towns began to spring up all over Lassen County. Bieber developed at the north end of the county, in rich farmland. Gold was discovered at Hayden Hill, and the small town developed to support the miners. Hayden Hill no longer exists: when the mining stopped, the townspeople left for other communities. Madeline was formed at the north end of another rich farming valley, and along the railroad tracks heading north to Alturas, California. This community still has about 50 people living in and around the town. In the 1890s many immigrant family groups arrived in the county, primarily coming from Lincolnshire and Herefordshire, England as well as the towns of Belgrade, Novi Sad, Niš and Kragujevac in Serbia. Several "Yankee" settlers arrived from Waldo County, Maine and Lincoln County, Maine as well.

During World War I, the area was heavily in favor of American entry into the war, and a disproportionate amount of volunteers from Lassen County signed up to take part in the war effort. A pro-German newspaper editor from San Francisco noted that "the inhabitants of Lassen County" were "sympathetic to Britain, hostile to Germany, and indifferent to France."

A narrow gauge railroad, the Nevada-California-Oregon Railway, ran through Lassen County from 1880 to 1927. The NCOR was the longest small gauge of the century. It was intended to connect Reno, Nevada, to the Columbia River, but only 238 miles (383 km) of track were laid, from Reno to Lakeview, Oregon.

In 1913, the Fernley & Lassen Railroad was built and it was used to export timber from the large forests of Lassen County. As this railroad was completed, the Red River Lumber Company set up shop, building the town of Westwood, California, to support its massive logging operation. Two other lumber mills followed the Red River Lumber Co. They built their mills in the county seat of Susanville. The Lassen Lumber and Box Company and the Fruit Growers Company both operated mills in Susanville for several decades.