A highly crystalline, 10 nm-sized red rust water oxidation catalyst composed of pure β-phase FeOOH(Cl) nanorods (ca. 3 × 13 nm) doped with Ni ions (β-FeOOH(Cl):Ni) and surface-modified with amorphous Ni(OH)2 (a-Ni(OH)2, at a Ni to Fe ratio of 22 at.%) was synthesized by a facile one-pot process at room temperature. The overpotential during the electrochemical oxygen evolution reaction (OER) over the β-FeOOH:Ni/a-Ni(OH)2 stacked nanorod anodes was 170 mV, and an OER current of 10 mA/cm2 was obtained at an overpotential of 430 mV in a 0.1 M KOH solution. X-ray absorption fine structure analysis, Mossbauer spectroscopy, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, X-ray diffraction and impedance spectroscopy suggested that surface modification with the a-Ni(OH)2 lowered the OER overpotential of β-FeOOH(Cl):Ni, resulting in the very high current density at low potential compared with Fe-rich oxide and oxyhydroxide electrodes reported previously. Mossbauer spectroscopy suggest...