Sodium thiosulphate is an inorganic compound with the formula Na2S2O3.xH2O. Typically it is available as the white or colourless pentahydrate. The solid is an efflorescent (loses water readily) crystalline substance that dissolves well in water.

Sodium thiosulphate is used in several pharmaceutical preparations and also has various medical properties. It is an important antidote used for treating cyanide poisoning. Apart from its medical uses, it also has applications in water treatment, neutralising bleach, leather tanning, gold extraction, photographic processing, and chemical heating pads.