Potassium Hydroxide is an inorganic compound with the formula KOH. Its common name is caustic potash. Along with sodium hydroxide (NaOH), this colourless solid is a prototypical strong base. It has many industrial and niche applications. Most applications exploit its reactivity toward acids and its corrosive nature. Potassium Hydroxide is noteworthy as the precursor to most soft and liquid soaps as well as numerous potassium-containing chemicals.

Potassium Hydroxide can be used interchangeably for a number of applications, although in industry, it is preferred because of its lower cost.

For precursor to other potassium compounds uses, many potassium salts are prepared by neutralization reactions involving Potassium Hydroxide. The potassium salts of carbonate, cyanide, permanganate, phosphate, and various silicates are prepared by treating either the oxides or the acids with Potassium Hydroxide. The high solubility of potassium phosphate is desirable in fertilizers.

For manufacture of biodiesels, although more expensive than using sodium hydroxide, Potassium Hydroxide works well in the manufacture of biodiesel by esterification of the free fatty acids in vegetable oil. Glycerin from Potassium Hydroxide biodiesel is useful as an inexpensive food supplement for livestock, once the toxic methanol is removed.

For manufacture of soft soaps, the saponification of fats with Potassium Hydroxide is used to prepare the corresponding "potassium soaps," which are softer than the more common sodium hydroxide-derived soaps. Because of their softness and greater solubility, potassium soaps require less water to liquefy, and can thus contain more cleaning agent than liquefied sodium soaps.

As an electrolyte, aqueous Potassium Hydroxide is employed as the electrolyte in alkaline batteries based on nickel-cadmium and manganese dioxide-zinc. Potassium Hydroxide is preferred over sodium hydroxide because its solutions are more conductive.