The Whitewater River is a small permanent stream in western Riverside County, California, with some upstream tributaries in southwestern San Bernardino County. The river's headwaters are in the San Bernardino Mountains, and it terminates at the Salton Sea in the Colorado Sonoran Desert. The area drained by the Whitewater River is part of the larger endorheic Salton Sea drainage basin.

Initially called Agua Blanco by early Spanish explorers, and later translated into English as white water, the river received its name for its milky appearance created by the silicate and lime sediments it carries. The community of Whitewater was named after the river, and became a key stop on the Bradshaw Trail stagecoach runs.

Sections of the river are federally protected as part of the National Wild and Scenic Rivers System; the river was designated in 2019. From the headwaters downstream to the San Gorgonio Wilderness, 23.5 miles, is classed as wild and administered by the U.S. Forest Service, and from the wilderness boundary downstream to the Sand to Snow National Monument boundary, 4.6 miles, is classed as recreational and administered by the Bureau of Land Management. The river canyon is an important collecting and gathering area for the Wanakik lineage of Cahuilla. It is also home to federally protected species like the southwestern willow flycatcher and the arroyo toad

The Whitewater River has three significant tributaries: the North, Middle, and South Forks, all within the Sand to Snow National Monument.

The North Fork begins in the subalpine zone at about 10,000 ft (3,000 m) on San Gorgonio Mountain and descends steeply southeast to the Middle Fork, which flows east through a wide arroyo. The South Fork flows northeast through a narrower wooded canyon, joining the Middle Fork lower down. The upper watershed is in the San Gorgonio Wilderness and San Bernardino National Forest, then it reaches land managed by the Bureau of Land Management. Below the confluences, the arroyo is at least 0.5 mi (0.80 km) wide, paved with accumulations of boulders, gravel, and sand brought down by floods and brushy except in stream channels cleared by floodwaters. Due to floods and shifting channels, there is almost no riparian forest development, except locally along unnamed minor tributaries with relatively stable channels.

The Pacific Crest Trail (PCT) enters the arroyo from the north and follows it downstream through Whitewater Canyon. A connecting trail links the PCT to the Whitewater Preserve, which serves as one of the primary public access points to this section of the river. A small population of wild but non-native Rainbow trout occurs locally in shaded reaches or cooler tributaries, but elevated summer temperatures limit their distribution along most of the stream.

Floods and debris flows periodically reshape the Whitewater Canyon section of the river. During the Valentine’s Day storm of February 2019 and again during Hurricane Hilary in August 2023, flooding damaged sections of Whitewater Canyon Road and affected access and trails in the canyon.

In contrast to the dry washes commonly visible in the Coachella Valley, the Whitewater River often flows above ground in the upper canyon of the San Bernardino Mountains, where it supports informal recreation such as hiking, wading, and wildlife viewing.