Aluminium sulfate is a chemical compound with the formula Al₂(SO₄)₃. It is soluble in water and is mainly used as a flocculating agent in the purification of drinking water and waste water treatment plants, and also in paper manufacturing.

Aluminium sulfate is sometimes referred to as a type of alum. Alums are double sulfate salts, with the formula AM(SO
4)
2·12H
2O, where A is a monovalent cation such as potassium or ammonium and M is a trivalent metal ion such as aluminium. The anhydrous form occurs naturally as a rare mineral millosevichite, found e.g. in volcanic environments and on burning coal-mining waste dumps. Aluminium sulfate is rarely, if ever, encountered as the anhydrous salt. It forms a number of different hydrates, of which the hexadecahydrate Al₂(SO₄)₃•16H₂O and octadecahydrate Al₂(SO₄)₃•18H₂O are the most common. The heptadecahydrate, whose formula can be written as [Al(H₂O)₆]₂(SO₄)₃•5H₂O, occurs naturally as the mineral alunogen.