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.

[6] Though the summer climate is hot, with limited shade and no water sources, there were no fires in the ATW for 110 years.

These include: The Agua Tibia Research Natural Area (ATRNA), located within the wilderness, comprises 480 acres (190 ha) of bigcone Douglas-fir—canyon live oak forest.

This population on the ATRNA is unique for its relatively great age, size, genetic purity, placement near the southern extent of the species' range, and for its remoteness and lack of disturbance by man.

[8][9] According to a U.S. government geologist writing in 1915, "Near the southern base of Agua Tibia Mountain and 25 miles in a direct line north of west from Warner Hot Springs, warm water (agua tibia), issues along a fault zone that is parallel to the one in Warner Valley.