The exploration of catalysts for oxygen evolution reaction (OER) using the advantages of interface engineering is an attractive strategy for achieving highly efficient electrochemical water splitting. In this study, Fe-doped multiphase nickel hydroxide (Fe-α,β-Ni(OH)2) was successfully prepared with a unique interface by modulating the Fe2+ and Fe3+ ratios in a feasible one-step hydrothermal process. This multiphase catalyst powder significantly contributed to the OER performance, with a low overpotential of 259 mV at 10 mA cm−2 (grown on carbon cloth substrates, 260 mV at 100 mA cm−2) and excellent catalytic stability during the 70-h OER process under extreme conditions. Based on the experimental and theoretical results, the unique interface between the Fe-α-Ni(OH)2 and Fe-β-Ni(OH)2 phases optimized the adsorption free energies of oxygen-containing intermediates and accelerated the formation of Ni3+ active species during the OER, leading to a large electrochemically active surface area (ECSA), which considerably enhanced the OER performance. This study provides novel insight into interface engineering and the design of high-performance OER catalysts based on earth-abundant transition-metal elements.