Lassen County, California is located in the high sierra region of northeastern California.

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, 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.