Despite significant advances in the development of highly efficient and robust oxygen evolution reaction (OER) electrocatalysts to replace noble-metal catalysts, commercializing OER catalysts with high catalytic activity for sustainable development still remains a great challenge. Especially, transition-metal Fe-based OER catalysts, despite their earth-abundant, cost-efficient, and environmentally benign superiorities over Co- and Ni-based materials, have received relatively insufficient attention because of their poor apparent OER activities. Herein, by rational design, we report Ni-modified pyrite (FeS2) spheres with yolk-shell structure that could serve as pre-electrocatalyst precursors to induce a highly active nickel-iron oxyhydroxide via in situ electrochemical topological transformation under the OER process. Notably, as confirmed by the results of X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations, Ni doping could effectively regulate the intrinsic electronic structure of FeS2 to realize a semiconductor-to-semimetal transition, which endows FeS2 with dramatically improved conductivity and water adsorption ability, providing prequisites for subsequent topological transformation. Moreover, systematic post-characterizations further reveal that the optimal Ni-FeS2-0.5 sample completely converts to amorphous Ni-doped FeOOH via an in situ electrochemical transformation with yolk-shell structure well-preserved under the OER conditions. The electronic structure modulation combined with electrochemical topotactic transformation strategies well stimulate the reactive Fe sites in Ni-FeS2-0.5, which show impressively low overpotentials of 250 and 326 mV to drive the current densities ( j) of 10 and 100 mA cm-2, respectively, and a Tafel slope as small as 34 mV dec-1 for the OER process. When assembled as a water electrolyzer for the overall water splitting, Ni-FeS2-0.5 can display a low voltage of 1.55 V to drive a current density of 10 mA cm-2, outperforming most of the transition-metal-based bifunctional electrocatalysts to date. This work may provide new insight into the rational design of other high-performance Fe-based OER electrocatalysts and inspire the exploration of cost-effective, ecofriendly electrocatalysts to meet the demand for future sustainable development.