An alum is a type of chemical compound that is usually a hydrated double sulfate salt of aluminum with the general formula XAl(SO4)2·12 H2O, where X is a monovalent cation such as potassium or ammonium. Alum can also refer to salts with the same formula and structure, except that aluminum is replaced by another trivalent metal ion like chromium III, and/or sulfur is replaced by another chalcogen like selenium. The most common of these analogs is chrome alum KCr(SO4)2·12 H2O. In most industries, the name "alum" (or "papermakers alum") is used to refer to aluminum sulfate, Al2(SO4)3·n H2O, which is used for most industrial flocculation.

Alum can be produced by precipitation from an aqueous solution. For example, in producing potassium alum, aluminum sulfate and potassium sulfate are dissolved in water, and then upon evaporation, the alum crystallizes out of the solution. Alums can also occur naturally in various minerals, such as kalinite, alunite, and leucite.

Alums have many uses, but they have been partly supplanted by aluminum sulfate itself, which is easily obtainable by treating bauxite ore with sulfuric acid. Some common uses of alum include baking and pickling, tanning leather, coagulation and flocculation in water treatment, and acting as an acidulating agent in cooking.

In addition to its chemical compound definition, "alum" can also refer to a graduate or former student of a particular school, college, or university. The word "alum" is an informal reference to either a male or female graduate, while "alumni" refers to a group of graduates that can be all male, all female, or mixed.