Barium sulfate (or barium sulphate), chemical formula BaSO₄, is an ionic compound and is essentially insoluble in water. Its melting point is 1580^o C and has a specific gravity of 4.5.

It is frequently used clinically as a radiocontrast agent for X-ray imaging and other diagnostic procedures. It is most often used in imaging of the GI tract. It is administered, orally or by enema, as a suspension of fine particles in an aqueous solution. Although barium is a heavy metal, and its water soluble compounds are often highly toxic, the extremely low solubility of barium sulfate protects the patient from absorbing harmful amounts of the metal.

Barium sulfate mixtures are used as white pigment for paints, when combined with zinc oxide (ZnO) it is called lithophone , when combined with sodium sulfate (Na₂SO₄) it is called blanc fixe . Barium sulfate is used as a filler in plastics and as a component of oil well drilling fluid to increase the density.

The mineral barite is composed largely of barium sulfate and is a common ore of barium.