Hydrochloric acid is a clear, colourless solution of hydrogen chloride (HCl) in water. It is a highly corrosive, strong mineral acid with many industrial uses. Hydrochloric acid is found naturally in gastric acid.

Historically called muriatic acid, and spirits of salt, hydrochloric acid was produced from vitriol (sulfuric acid) and common salt. It first appeared during the Renaissance, and then it was used by chemists such as Glauber, Priestley and Davy in their scientific research.

With major production starting in the Industrial Revolution, hydrochloric acid is used in the chemical industry as a chemical reagent in the large-scale production of vinyl chloride for PVC plastic, and MDI/TDI for polyurethane. It has numerous smaller-scale applications, including household cleaning, production of gelatin and other food additives, descaling, and leather processing. About 20 million tonnes of hydrochloric acid are produced annually.

Hydrochloric acid was known to European alchemists as spirits of salt or acidum salis (salt acid). Both names are still used, especially in non-English languages, such as German: Salzsäure and Dutch: Zoutzuur. Gaseous HCl was called marine acid air. The old (pre-systematic) name muriatic acid has the same origin (muriatic means "pertaining to brine or salt"), and this name is still sometimes used.