Tuolumne County, officially the County of Tuolumne, is a county located in the U.S. state of California. As of the 2020 census, the population was 55,620. The county seat and only incorporated city is Sonora. Tuolumne County comprises the Sonora, CA Micropolitan Statistical Area.

One of California’s original 27 counties, Tuolumne was organized in 1850.

Prior to the official naming of counties by the state, Tuolumne was sometimes referred to as Oro County.

The original lines of Tuolumne County were not long established. In 1854 and 1855 the portion of Tuolumne County that extended west into the San Joaquin Valley was reorganized as Stanislaus County. In 1864 a number of the original counties including Tuolumne contributed lands that would lead to the establishment of Alpine County to the northeast. With the State’s Adoption of the Political Code in 1872 the current boundaries of Tuolumne County were largely established as shown in the maps below.