Actinium(III) oxide is a chemical compound containing the rare radioactive element actinium. It has the molecular formula Ac<sub>2</sub>O<sub>3</sub>. It is similar to its corresponding lanthanum compound, lanthanum(III) oxide, and contains actinium in the oxidation state +3. Actinium oxide is not to be confused with Ac<sub>2</sub>O (acetic anhydride), where Ac is an abbreviation for acetyl instead of the symbol of the element actinium.