The Salton Sea is a shallow, landlocked, highly saline body of water in Riverside and Imperial counties at the southern end of the U.S. state of California. It lies on the San Andreas Fault within the Salton Trough, which stretches to the Gulf of California in Mexico.

Over millions of years, the Colorado River had flowed into the Imperial Valley and deposited alluvium (soil), creating fertile farmland, building up the terrain, and constantly moving its main course and river delta. For thousands of years, the river alternately flowed into the valley, or diverted around it, creating either a salt lake called Lake Cahuilla, or a dry desert basin, respectively. When the Colorado River flowed into the valley, the lake level depended on river flows and the balance between inflow and evaporative loss. When the river diverted around the valley, the lake dried completely, as it did around 1580. Hundreds of archaeological sites have been found in this region, indicating possibly long-term Native American villages and temporary camps.

The current lake was formed from an inflow of water from the Colorado River in 1905. Beginning in 1900, an irrigation canal was dug from the Colorado River to the old Alamo River channel to provide water to the Imperial Valley for farming. The headgates and canals sustained a buildup of silt, so a series of cuts were made in the bank of the Colorado River to further increase the water flow. Water from spring floods broke through a canal head-gate diverting a portion of the river flow into the Salton Basin for two years before repairs were completed. The water in the formerly dry lake bed created the modern lake, which is about 15 by 35 miles (24 by 56 km) at its widest and longest. A 2023 report puts surface area at 318 square miles.

During the early 20th century the lake would have dried up, except that farmers used generous amounts of Colorado River water for irrigation and let the excess flow into the lake. In the 1950s and into the 1960s, the area became a resort destination, and communities grew with hotels and vacation homes. Birdwatching was also popular as the wetlands were a major resting stop on the Pacific Flyway. In the 1970s, scientists issued warnings that the lake would continue to shrink and become more inhospitable to wildlife. In the 1980s, contamination from farm runoff promoted the outbreak and spread of wildlife diseases. Massive die-offs of the avian populations have occurred, especially after the loss of several species of fish on which they depend. Salinity rose so high that large fish kills occurred, often blighting the beaches of the sea with their carcasses. Tourism was drastically reduced.

After 1999, the lake began to shrink as local agriculture used the water more efficiently, so less runoff flowed into the lake. As the lake bed became exposed, the winds sent clouds of toxic dust into nearby communities. Smaller amounts of dust reached into the Los Angeles area and people there could sometimes smell an odor coming from the lake. The state is mainly responsible for fixing the problems. California lawmakers pledged to fund air-quality management projects in conjunction with the signing of the 2003 agreement to send more water to coastal cities. Local, state and federal bodies all had found minimal success dealing with the dust, dying wildlife, and other problems for which warnings had been issued decades before. At the beginning of 2018, local agencies declared an emergency and, along with the state, funded and developed the Salton Sea Management Program.

In 2020, Palm Springs Life magazine summarized the ecological situation as "Salton Sea derives its fame as the biggest environmental disaster in California history".