Materials with high photoluminescence (PL) intensity can potentially be used in optical and electronic devices. Although the PL properties of bismuth­(III) oxide with a monoclinic crystal structure (α-Bi₂O₃) have been explored in the past few years, methods of increasing PL emission intensity and information relating PL emission to structural defects are scarce. This research evaluated the effect of a pressure-assisted heat treatment (PAHT) on the PL properties of α-Bi₂O₃ with a needlelike morphology, which was synthesized via a microwave-assisted hydrothermal (MAH) method. PAHT caused an angular increase between the [BiO₆]–[BiO₆] clusters of α-Bi₂O₃, resulting in a significant increase in the PL emission intensity. The Raman and XPS spectra also showed that the α-Bi₂O₃ PL emissions in the low-energy region (below ∼2.1 eV) are attributed to oxygen vacancies that form defect donor states. The experimental results are in good agreement with first-principles total-energy calculations that were carried out within periodic density functional theory (DFT).