Hydraulic mining is a form of mining that uses high-pressure jets of water to dislodge rock material or move sediment. In the placer mining of gold or tin, the resulting water-sediment slurry is directed through sluice boxes to remove the gold or tin. It is also used in mining kaolin and coal.

Hydraulic mining developed from ancient Roman techniques that used water to excavate soft underground deposits. Its modern form, using pressurized water jets produced by a nozzle called a "monitor", came about in the 1850s during the California Gold Rush in the United States. Though successful in extracting gold-rich minerals, the widespread use of the process resulted in extensive environmental damage, such as increased flooding and erosion, and sediment blocking waterways and covering farm fields. These problems led to its legal regulation. Hydraulic mining has been used in various forms around the world.

Hydraulic mining had its precursor in the practice of ground sluicing, a development of which is also known as "hushing", in which surface streams of water were diverted so as to erode gold-bearing gravels. This technique was developed in the first centuries BC and AD by Roman miners to erode away alluvium. The Romans used ground sluicing to remove overburden and the gold-bearing debris in Las Médulas of Spain, and Dolaucothi in Great Britain. Later archaeological work in the North Pennines of northern England identifies hushing as a hydraulic mining technique used in post-medieval lead-mining landscapes, with evidence that it was widespread there during the 18th century.

Water was used on a large scale by Roman engineers in the first centuries BC and AD when the Roman Empire was expanding rapidly in Europe. Using a process later known as hushing, the Romans stored a large volume of water in a reservoir immediately above the area to be mined; the water was then quickly released. The resulting wave of water removed overburden and exposed bedrock. Gold veins in the bedrock were then worked using a number of techniques, and water power was used again to remove debris. The remains at Las Médulas and in surrounding areas show badland scenery on a gigantic scale owing to hydraulicking of the rich alluvial gold deposits.

Las Médulas is now a UNESCO World Heritage Site. The site shows the remains of at least seven large aqueducts of up to 30 miles (48 km) in length feeding large supplies of water into the site. The gold-mining operations were described in vivid terms by Pliny the Elder in his Natural History published in the first century AD. Pliny was a procurator in Hispania Terraconensis in the 70s AD and witnessed the operations himself. At Dolaucothi in South Wales, archaeological survey of the Roman gold mine's surface remains documented a complex aqueduct system above the main workings; the site is identified by the National Trust as the only known Roman gold mine in Great Britain.

The modern form of hydraulic mining, using jets of water directed under very high pressure through hoses and nozzles at gold-bearing upland paleogravels, was first used by Edward Matteson near Nevada City, California in 1853 during the California Gold Rush. Matteson used canvas hose which was later replaced with crinoline hose by the 1860s. In California, hydraulic mining often brought water from higher locations for long distances to holding ponds several hundred feet above the area to be mined. California hydraulic mining exploited gravel deposits, making it a form of placer mining.

Early placer miners in California discovered that the more gravel they could process, the more gold they were likely to find. Instead of working with pans, sluice boxes, long toms, and rockers, miners collaborated to find ways to process larger quantities of gravel more rapidly. Hydraulic mining became the largest-scale, and most devastating, form of placer mining. Water was redirected into an ever-narrowing channel, through a large canvas hose, and out through a giant iron nozzle, called a "monitor". The extremely high pressure stream was used to wash entire hillsides through enormous sluices.

By the early 1860s, while hydraulic mining was at its height, small-scale placer mining had largely exhausted the rich surface placers, and the mining industry turned to hard rock (called quartz mining in California) or hydraulic mining, which required larger organizations and much more capital. By the mid-1880s, it is estimated that 11 million ounces of gold (worth approximately US$7.5 billion at mid-2006 prices) had been recovered by hydraulic mining .