Antimony trioxide is the inorganic compound with the formula Sb2O3. It is the most important commercial compound of antimony. It is found in nature as the minerals valentinite and senarmontite. Like most polymeric oxides, Sb2O3 dissolves in aqueous solutions only with hydrolysis.

Antimony trioxide is an amphoteric oxide, dissolving in alkaline solution to give antimonites and in acid solution to given a range of polyantimonous acids. It can be readily oxidised to antimony pentoxide and related antimony(V) compounds, but it is also easily reduced to antimony, sometimes with production of stibine.