Agua Mansa is a former settlement in an unincorporated area of San Bernardino County, near Colton, California, United States. Once the largest settlement in San Bernardino County, it is now a ghost town. Only the cemetery remains. The town was established in 1842 in early California Alta California. It was on the Santa Ana River, across from the era settlement of La Placita. Agua Mansa and La Placita were the first non-native settlements in the San Bernardino Valley.