A quenchable new high-pressure–high-temperature modification of antimony(III) oxide, γ-Sb₂O₃, has been obtained at hydrostatic pressures of 9–11 GPa and temperatures of 573–773 K. Its crystal structure has been determined from high-resolution synchrotron powder diffraction data. γ-Sb₂O₃consists of three-dimensionally cross-linked infinite chains of SbO₃Eunits (E= lone pair) with the chains forming tetragonal rod-packing. The underlying topology of γ-Sb₂O₃(3,3T8) is found very rarely in inorganic structures; it is realised only for the polyanion [Si₄O₄N₆]¹⁰⁻that occurs in the Ce₄(Si₄O₄N₆)O structure type. The structural relation to the two previously known polymorphs of Sb₂O₃at ambient pressure, valentinite and senarmontite is discussed.