Agua Tibia Wilderness, Nature reserve in Riverside County, United States.

Agua Tibia Wilderness is a nature reserve in Southern California featuring steep canyons, mountains, and a range of plant communities across its protected terrain. The landscape displays varied habitats shaped by elevation changes and different soil types throughout the area.

The area was first protected as the Agua Tibia Primitive Area, then added to the National Wilderness Preservation System in January 1975 by Public Law 93-632, passed by Congress. From that designation until 1984 it was San Diego County's only officially designated wilderness area.

The name Agua Tibia is Spanish for "warm water", taken from the warm spring described in 1915 near the southern base of Agua Tibia Mountain. A government geologist reported water rising at 92 °F (33 °C) in a marshy area of about an acre, with bubbles continually rising and a distinctly sulphureted smell. It was piped from a board-curbed pool to tubs in a nearby house for bathing and laundry.

Visitors need to obtain required permits before entering and must follow marked trails to protect sensitive areas from damage. Motorized vehicles and drones are not allowed, so plan for hiking or horseback riding as your means of exploration.

The 480-acre Agua Tibia Research Natural Area protects a stand of bigcone Douglas-fir and canyon live oak. The bigcone Douglas-fir is a relict species endemic to Southern California, and this population is noted for its age, size, genetic purity and position near the southern limit of the species' range.