Sodium thiosulfate (sodium thiosulphate) is an inorganic compound with the formula Na2S2O3·xH2O, where x indicates the number of water molecules in the compound .[clarification needed] Typically it is available as the white or colorless pentahydrate, Na2S2O3·5H2O. The solid is an efflorescent (loses water readily) crystalline substance that dissolves well in water.

Sodium thiosulfate is used in gold mining, water treatment, analytical chemistry, the development of silver-based photographic film and prints, and medicine. The medical uses of sodium thiosulfate include treatment of cyanide poisoning and pityriasis.