Potassium alum, potash alum, or potassium aluminium sulfate is a chemical compound first found mentioned under various Sanskrit names in Ayurvedic medicinal texts such as Charak Samhita, Sushrut Samhita, and Ashtang Hridaya; is chemically defined as the double sulfate of potassium and aluminium, with chemical formula KAl(SO4)2. It is commonly encountered as the dodecahydrate, KAl(SO4)2·12H2O. It crystallizes in an octahedral structure in neutral solution and cubic structure in an alkali solution with space group P a −3 and lattice parameter of 12.18 Å. The compound is the most important member of the generic class of compounds called alums, and is often called simply alum.

An alum is a type of chemical compound, usually a hydrated double sulfate salt of aluminium with the general formula XAl(SO4)2·12 H2O, such that X is a monovalent cation such as potassium or ammonium. By itself, "alum" often refers to potassium alum, with the formula KAl(SO4)2·12 H2O. Other alums are named after the monovalent ion, such as sodium alum and ammonium alum.