Agua Tibia Wilderness (ATW) is a 17,961-acre (72.69 km2) protected area in Riverside and San Diego counties, in the U.S. state of California. It is mostly within the Palomar Ranger District of the Cleveland National Forest. The area was originally protected as the Agua Tibia Primitive Area until January 1975 when it was added to the National Wilderness Preservation System with the passage of Public Law 93-632 by the United States Congress. Between its inception and 1984, the ATW was San Diego County's only officially designated wilderness area. The Spanish name, Agua Tibia, translates as warm water.