An alum () is a type of chemical compound, usually a hydrated double sulfate salt of aluminium with the general formula XAl(SO
₄)
₂·12 H
₂O, where X is a monovalent cation such as potassium or ammonium. By itself, "alum" often refers to potassium alum, with the formula KAl(SO
₄)
₂·12 H
₂O. Other alums are named after the monovalent ion, such as sodium alum and ammonium alum.

The name "alum" is also used, more generally, for salts with the same formula and structure, except that aluminium 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(SO
₄)
₂·12 H
₂O.

In most industries, the name "alum" (or "papermaker's alum") is used to refer to aluminium sulfate, Al
₂ (SO
₄)
₃·n H
₂O, which is used for most industrial flocculation^: 766 (the variable n is an integer whose size depends on the amount of water absorbed into the alum). In medicine, "alum" may also refer to aluminium hydroxide gel used as a vaccine adjuvant.