The rational design and fabrication of junction-based photocatalysts is of great importance to enhance the photocatalytic efficiency of water splitting. Herein, a new heterophase junction photocatalyst consisting of cubic bixbyite and rhombohedral corundum In₂O₃ (c-In₂O₃ and h-In₂O₃) was readily prepared by directly annealing an InOOH precursor in an inert atmosphere. The resulting c/h-In₂O₃ heterophase junction photocatalyst showed an optimal hydrogen production rate of 0.73 mmol g⁻¹ h⁻¹ in a 6 h period reaction from alkaline formaldehyde solution in the presence of molecular oxygen under visible light irradiation, which was approximately 5.6 and 3.4 times higher than that of single-component c-In₂O₃ and h-In₂O₃, respectively. The superior photocatalytic activity of the c/h-In₂O₃ heterophase junction was partially attributed to its appropriate band gap and band positions. Spectroscopic characterization and photoelectrochemistry experiments indicated that the heterophase junction photocatalyst could effectively enhance charge separation and transfer abilities between h-In₂O₃ and c-In₂O₃. Furthermore, molecular O₂ was found to play a key role in mediating the photocatalytic H₂ evolution from formaldehyde oxidation as a catalytic electron scavenger, and O₂ was not consumed throughout the reaction. Thus, the photocatalytic activity was also greatly enhanced because the adsorption ability of oxygen species was effectively improved after the formation of heterophase junctions.