Potassium ( /pɵˈtæsiəm/ po-TAS-ee-əm) is the chemical element with the symbol K (from Neo-Latin kalium) and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.

Because potassium and sodium are chemically very similar, it took a long time before their salts were differentiated. The existence of multiple elements in their salts was suspected from 1702, and this was proven in 1807 when potassium and sodium were individually isolated from different salts by electrolysis. Potassium in nature occurs only in ionic salts. As such, it is found dissolved in seawater (which is 0.04% potassium by weight), and is part of many minerals.

Most industrial chemical applications of potassium employ the relatively high solubility in water of potassium compounds, such as potassium soaps. Potassium metal has only a few special applications, being replaced in most chemical reactions with sodium metal.