[en] Developing efficient catalysts to accelerate the rate of oxygen evolution reaction (OER) is critical for photocatalytic water-splitting. In this work, metallic Ir, IrO_x(OH)_y, and core–shell Ir@IrO_x(OH)_y were synthesized and employed as OER catalysts for photocatalytic water oxidation. It was found that the Ir@IrO_x(OH)_y core–shell heterostructure catalyst showed the best photocatalytic performance among these three catalysts, with the oxygen evolution rate as high as 59.63 mmol g⁻¹ h⁻¹. Detailed investigations revealed that the excellent photocatalytic activity of Ir@IrO_x(OH)_y could be attributed to both the outstanding intrinsic activity of IrO_x(OH)_y shell and the efficient electron transfer between the photosensitizer and catalyst. (paper)