The San Gabriel River is a mostly-urban waterway flowing 58 miles (93 km) southward through Los Angeles and Orange Counties, California, in the United States. It is the central of three major rivers draining the Greater Los Angeles area, the others being the Los Angeles River and Santa Ana River. The river's watershed stretches from the rugged San Gabriel Mountains to the heavily developed San Gabriel Valley and a significant part of the Los Angeles coastal plain, emptying into the Pacific Ocean between the cities of Long Beach and Seal Beach.

The San Gabriel once ran across a vast alluvial flood plain, its channels shifting with winter floods and forming extensive wetlands along its perennial course, a relatively scarce source of fresh water in this arid region. The Tongva and their ancestors inhabited the San Gabriel River basin for thousands of years at villages like Puvunga, relying on the abundant fish and game in riparian habitats. The river is named for the nearby Mission San Gabriel Arcángel, established in 1771 during the Spanish colonization of California. Its water was heavily used for irrigation and ranching by Spanish, Mexican, and American settlers before urbanization began in the early 1900s, eventually transforming much of the watershed into industrial and suburban areas of greater Los Angeles.

Severe floods in 1914, 1934, and 1938 spurred Los Angeles County, and later the federal government, to build a system of dams and debris basins and to channelize much of the lower San Gabriel River with riprap or concrete banks. There is also an extensive system of spreading grounds and other works to capture stormwater runoff and conserve it for urban use. Approximately one-third of the water used in southeast Los Angeles County today comes from the river.

The upper San Gabriel has been intermittently mined for gold since the 1860s, and its deep gravel bed has been an important source of construction aggregate since the early 1900s. The river is also a popular recreation area, with parks and trails in the many flood basins along its course. The headwaters of the San Gabriel River have retained their natural character and are a popular attraction of the Angeles National Forest.

The San Gabriel River basin drains a total of 689 square miles (1,780 km²) and is located between the watersheds of the Los Angeles River to the west, the Santa Ana River to the east, and the Mojave Desert to the north. The watershed is divided into three distinct sections. The northern third, located within the Angeles National Forest of the San Gabriel Mountains, is steep and mountainous; it receives the most precipitation of any part of the basin – 33 inches (840 mm) per year – and as a result is the source of nearly all the natural runoff. Elevations reach up to 10,064 feet (3,068 m) at Mount San Antonio (Mount Baldy), the highest point of the range. During the winter, many elevations above 6,000 feet (1,800 m) are covered in snow.

The middle third, the San Gabriel Valley, and the southern third, the coastal plain of the Los Angeles Basin, are separated by the Puente Hills and Montebello Hills. With the exception of some recreation areas and lands set aside for flood control, the valleys are almost entirely urbanized. Approximately 2 million people live in the watershed, divided among 35 incorporated cities. Rainfall is slightly higher in the San Gabriel Valley than the coastal plain due to its proximity to the mountains; however, the climate as a whole is very arid, with only moderate precipitation in winter and nearly none in summer. The lower watershed consists primarily of alluvial plains that once experienced seasonal flooding from the San Gabriel River, creating vast swamps and wetlands. Today, very little of this original environment remains.

The San Gabriel is one of the largest natural streams in Southern California, but its discharge varies widely from year to year. Between 1895 and 1957 the mean unimpaired runoff at Azusa was estimated at 114,000 acre-feet (141,000,000 m³), with a range from 9,600 to 410,000 acre-feet (11,800,000 to 505,700,000 m³). Historically, the San Gabriel River reached its highest flows in the winter and spring, with runoff dropping significantly after early June before rising again with November or December storms. Today, the flow of the San Gabriel River has been dried up in places by dams, diversions, and groundwater recharge operations, and increased in other sections by wastewater run-off.

The East Fork, 17 miles (27 km) long, is the largest headwater of the San Gabriel River; the U.S. Geological Survey considers it part of the main stem. However, it is colloquially known as the "East Fork" to distinguish it from the West Fork of the San Gabriel. Its furthest tributary, the Prairie Fork, originates at 9,648-foot (2,941 m) Pine Mountain in the Sheep Mountain Wilderness to the southwest of Wrightwood. Draining a high, remote, subalpine valley characterized by extensive meadows, it flows west to join with Vincent Gulch, below which the stream is officially known as the East Fork. Here it turns abruptly south, flowing through a steep, rugged canyon. It is joined from the east by the Fish Fork, which originates on the northwest slopes of Mount Baldy.