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We report a joint experimental and theoretical study of the structural and vibrational properties of synthetic sphaerobismoite (β-Bi₂O₃) at high pressures in which room-temperature angle-dispersive X-ray diffraction (XRD) and Raman scattering measurements have been complemented with ab initio total-energy and lattice dynamics calculations. Striking changes in Raman spectra were observed around 2 GPa, whereas X-ray diffraction measurements evidence no change in the tetragonal symmetry of the compound up to 20 GPa; however, a significant change exists in the compressibility when increasing pressure above 2 GPa. These features have been understood by means of theoretical calculations, which show that β-Bi₂O₃ undergoes a pressure-induced isostructural phase transition near 2 GPa. In the new isostructural β′ phase, the Bi³⁺ and O^2– environments become more regular than those in the original …