The formation of oxygen vacancies (Vö) in n-type semiconductors is a key strategy for improving the performance of metal-oxide-based photoanodes. Whereas Vö has traditionally been created by gas- or liquid-phase treatments, here we report a solid-state reduction technique termed the “low-temperature thermite reaction” (LTTR), which is effective for various metal oxides and solid reductants. In the case of ZnFe2O4 (ZFO), the LTTR increases charge-carrier density and bulk charge-separation efficiency by ∼100-fold and 2∼4-fold, respectively, for ZFO with an Fe reductant relative to pristine ZFO. The photocurrent densities for sacrificial reagent and water oxidation (1.8 and 1.6 mA/cm2 at 1.23 VRHE, respectively) achieved here represent the highest values reported for ZFO photoanodes. Also, a ZFO-lead halide perovskite solar cell tandem water-splitting cell demonstrated an unbiased solar-to-hydrogen efficiency of 1.85%. The LTTR is applicable to large-area (25 cm2) photoanodes under ambient atmosphere. Thus, the LTTR could become a more effective and versatile technique than conventional ones.