Agua Tibia Wilderness is a protected backcountry in Southern California spanning parts of Riverside and San Diego counties within Cleveland National Forest. First protected as a primitive area and added to the National Wilderness Preservation System in 1975, it was San Diego County’s only officially designated wilderness until 1984. The rugged Peninsular Ranges landscape includes Agua Tibia Mountain and is known for harsh, dry summer conditions with no permanent streams or reliable water. Despite major fires in recent decades, the area remains important for rare plants, a distinctive bigcone Douglas-fir research forest, and warm springs near the mountain’s southern base that gave the wilderness its name.