The introduction of oxygen vacancies into zirconia is an effective strategy for enhancing its light absorption ability and photocatalytic performance. However, the cost-efficient preparation of oxygen-deficient zirconia (ZrO_2−x) remains challenging, which severely limits its broad application. In this study, flash sintering treatment was used to fabricate ZrO_2−x bulk in very short time of 90 s. Oxygen vacancies were introduced into ZrO₂ bulk through electrochemical reduction reactions. The as-prepared black ZrO_2−x exhibited excellent optical absorption capability, a small band gap (2.09 eV for direct and 1.67 eV for indirect), and a reduced conduction band energy, which is ascribed to the generation of oxygen vacancies and reduction of Zr cations. The as-prepared ZrO_2−x showed remarkable photocatalytic activity due to excellent solar light absorption and low recombination rate of electron‒hole pairs. Flash sintering treatment provides a cost-efficient approach for rapidly fabricating ZrO_2−x bulk materials with high concentrations of oxygen vacancies, which can also be applied to other materials.