Agua Fria (; Spanish for "cold water") is a ghost town in Mariposa County, California that served as the first county seat of Mariposa County from February 18, 1850, to November 10, 1851. The settlement was destroyed by fire in 1866 and was not rebuilt.
The site is designated California Historical Landmark No. 518. A marker stands on California State Route 140, west of Mariposa.
Sonoran miners settled the area in the early summer of 1849 during the California Gold Rush. The settlement was divided into Lower Agua Fria and Upper Agua Fria; the name derived from two springs of cold water located about a quarter mile below Lower Agua Fria. It may have been in this area that John C. Frémont's men discovered gold in 1849 on his Rancho Las Mariposas.
On February 18, 1850, the California Legislature enacted legislation designating Agua Fria as the seat of justice for the newly created Mariposa County, which at that time encompassed roughly one-sixth of California's total area, including what are now Merced, Madera, Fresno, Tulare, Kings, and Kern counties. A post office was established on October 7, 1851. An election held on September 3, 1851, moved the county seat to Mariposa, effective November 10, 1851.
Local placer gold deposits were largely exhausted by the early 1850s, and miners departed for other camps, including lode-mining areas such as Whitlock and Mount Bullion. A fire on June 22, 1866, destroyed the remaining settlement. Contemporary reporting in the Mariposa Gazette stated that the fire originated in a Chinese church and consumed approximately 75 buildings, leaving one structure standing. The post office closed in 1862. No structures from the original settlement survive.
Agua Fria is a ghost town with little remaining visible at the surface. The site is private property and is accessible via Agua Fria Road toward Mount Bullion.