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Bismuth oxide (Bi₂O₃), a yellow semiconductor ceramic, is considered as an advanced photoactive energy material, but its activity still needs further improvement for practical applications. In this study, black Bi₂O₃ with nanocrystalline structure and large fraction of oxygen vacancies is synthesized by mechanical straining via the high-pressure torsion (HPT) method. The black oxide exhibits enhanced light absorbance under both UV and visible lights. It shows a bipolar photocurrent behavior with up to six times higher photocurrent density compared with the unprocessed yellow Bi₂O₃. This study shows the high potential of severely-strained black Bi₂O₃ for photovoltaic, photoconductivity and photocatalytic applications.