Road salt (also known as de-icing salt, rock salt, or snow salt) is a salt used mainly as an anti-slip agent in winter road conditions, but also to prevent dust and snow build-up on roads. Various kinds of salts are used as road salt, but calcium chloride and sodium chloride (rock salt) are among the most common. The more expensive magnesium chloride is generally considered safer, but is not as widely used because of its cost and effect on structural integrity. When used in its solid form, road salt is often pre-wet to accelerate the ice-melting process. Road salts have been having adverse effects on the environment, including decreasing biodiversity, contaminating of water sources, and increasing soil salinity.

The use of salt for deicing roads began in the United States in the late 1930s, when New Hampshire experimented with spreading granular sodium chloride on roads in 1938. By the winter of 1941–1942, New Hampshire formally adopted a statewide salt-spreading policy; about 5,000 tons of salt were applied on U.S. highways that season. Before the adoption of road salt in New Hampshire, road maintenance in winter typically relied on plowing and spreading abrasives (sand, cinders) for traction, with salt only used occasionally (e.g. to slow freezing in stored sand piles).

In the post-war era, as the U.S highway networks expanded and the “bare pavement” standard (expectation that roads be cleared quickly) became dominant, salt usage increased greatly. During the 1950s and 1960s, U.S. salt consumption doubled roughly every five years, climbing from about 1 million tons in the mid-1950s to nearly 10 million tons less than a decade later. Supply sources developed along with the demand, large underground rock-salt deposits (formed by the evaporation of ancient seas) supplied much of the salt. The Detroit salt mine (first discovered in 1895) became a prominent example as the mines expanded over the 20th century and supported local deicing operations. Detroit itself was among the earliest cities to apply salt to its roads (circa 1940) believed to be due to its proximity to the resource.

In later decades, salt use stabilized (on the order of tens of millions of tons per year in the U.S.), and attention has shifted to optimizing application methods and mitigating collateral impacts. More recently, occasional supply constraints and environmental concerns have spurred experimentation with reduced-salt strategies and alternative deicers (e.g. prewetting, salt–organic blends, calcium or magnesium salts).

Road salt and brine are generally spread using a winter service vehicle called a salt spreader. Salt spreaders are typically added to trucks, loaders, or in the case of brine, tankers. The salt is stored in the large hopper on the rear of the vehicle, with a wire mesh over the top to prevent foreign objects from entering the spreading mechanism and hence becoming jammed. The salt is generally spread across the roadway by an impeller, attached by a hydraulic drive system to a small onboard engine. However, until the 1970s, it was often spread manually either by workers shoveling salt from trucks or by smaller wheelbarrow-like vehicles, the latter still being used today for personal use. Some older spreading mechanisms still require it to be manually loaded into the impeller from the hopper.

Salt for use of melting ice and snow works through a phenomenon called freezing-point depression, the lowering of a substance's freezing point after the addition of solutes. When road salt is added to roads, aside from providing better friction for vehicles on the road, it also dissolves in the water of the ice, resulting in a lower freezing point. As long as the temperature is above this freezing point, this in turn results in the ice melting. Because of this, ordinary rock salt is only effective down to a range of −6 to −10 °C (21 to 14 °F). At colder temperatures, it can have the opposite effect. Road salt is sometimes used even in colder conditions, if milder weather is expected. In very cold and dry weather, the road surface becomes rough and the need for de-icing is reduced. However, during extreme cold and rain, the roads can become extremely difficult to pass and, in some cases, roads may need to be closed to traffic.

Sodium chloride is by far the most common kind of road salt. This is mainly due to its widespread use and low cost, and thanks to its large industrial infrastructure, it is used in many industrial and consumer applications. While it is common and inexpensive, its effective temperature range usually does not fall below −6 to −10 °C (21 to 14 °F), and under these temperatures, it is often counter-productive. When used in large quantities, it can also disrupt local ecosystems by heightening the salinity of bodies of water and the soil. Further, rock salt's abrasive nature erodes concrete or asphalt if used heavily.

Calcium chloride is less common compared to sodium chloride. While it is slightly more expensive, it can cover a far larger area and melts ice almost three times quicker. It has recently started rising in popularity since it is not as environmentally damaging as sodium chloride, and also because of its heightened effectiveness at clearing ice.