Barium carbonate (BaCO₃), also known as witherite, is a chemical compound used in rat poison, bricks, and cement.

Witherite crystallizes in the orthorhombic system. The crystals are invariably twinned together in groups of three, giving rise to pseudo-hexagonal forms somewhat resembling bipyrarnidal crystals of quartz, the faces are usually rough and striated horizontally. The color is dull white or sometimes greyish, the hardness is 3.5, and the specific gravity is 4.3.

The mineral is named after W. Withering, who in 1784 recognized it to be chemically distinct from barytes. It occurs in veins of lead ore at Hexham in Northumberland, Alston in Cumbria, Anglezarke , near Chorley in Lancashire, and a few other localities. Witherite is readily altered to barium sulfate by the action of water containing calcium sulphate in solution, and crystals are therefore frequently encrusted with harytes. It is the chief source of barium salts, and is mined in considerable amounts in Northumberland. It is used for the preparation of rat poison, in the manufacture of glass and porcelain, and formerly for refining sugar.