Barium borate is an inorganic compound, a borate of barium with a chemical formula BaB₂O₄ or Ba(BO₂)₂. It is available as a hydrate or dehydrated form, as white powder or colorless crystals. The crystals exist in the high-temperature α phase and low-temperature β phase, abbreviated as BBO; both phases are birefringent, and BBO is a common nonlinear optical material.