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Among insulating transition-metal oxides, ZrO2has the potential to be a photocatalyst; however, its wide bandgap (∼5 eV) interferes with its ability to absorb sufficient light. Defect engineering—that is, the creation of oxygen vacancies (VOs) and/or Zr3+ions, is a promising strategy for adjusting the bandgap of ZrO2for photocatalytic applications. This study demonstrates that the included VOs play an important role in boosting photocatalytic activity. In addition, the crystal phase of ZrO2(e.g., monoclinic or tetragonal) strongly influences both VOformation and photocatalytic activity. After a Pt cocatalyst was deposited onto ZrO2with a single crystal phase (i.e., monoclinic (m-) or tetragonal (t-)), the specimens were reduced at 200, 400, and 600 °C to obtain photocatalysts with different amounts of VOs. As-synthesized m-ZrO2possessed Zr3+ions in the absence of VOs, and the reduction treatment promoted VOformation accompanied by the disappearance of Zr3+. The as-synthesized t-ZrO2originally included VOs, and their amount in Pt/t-ZrO2−xincreased with increasing reduction temperature. Pt/t-ZrO2−xpossessed a greater amount of VOs than Pt/m-ZrO2−xreduced at the same temperature, suggesting that VOformation occurs more easily in t-ZrO2than in m-ZrO2. Photocatalytic activity increased with increasing reduction temperature, which is the same tendency observed for the VOamount. In a comparison of the crystal phases, Pt/m-ZrO2−xexhibited relatively higher activity than Pt/t-ZrO2−xdespite its fewer VOs, suggesting that the VOformed in the monoclinic ZrO2contributes to the enhancement of the photocatalytic activity more efficiently than in the tetragonal one.