A new polymorphic form of bismuth oxide (Bi2O3) is synthesized by a high-pressure technique (6 GPa and 880 degrees C, and subsequently slow cooling). This phase can exist under ambient conditions, but it is fairly unstable. The crystal structure is determined to be the A-type rare-earth sesquioxide structure that is isotypic to La2O3. Hexagonal cell parameters are measured to be a = 3.878(1) Angstrom and c = 6.303(1) Angstrom. The pressure effect on a polarizable 6s2 lone pair character of Bi3+ is discussed.