The rational construction of earth‐abundant and advanced electrocatalysts for oxygen evolution reaction (OER) is extremely desired and significant to seawater electrolysis. Herein, by directly etching Ni₃S₂ nanosheets through potassium ferricyanide, a novel self‐sacrificing template strategy is proposed to realize the in situ growth of NiFe‐based Prussian blue analogs (NiFe PBA) on Ni₃S₂ in an interfacial redox reaction. The well‐designed Ni₃S₂@NiFe PBA composite as precursor displays a unique spherical magic cube architecture composed of nanocubes, which even maintains after a phosphating treatment to obtain the derived Ni₃S₂/Fe‐NiP_x on nickel foam. Specifically, in alkaline seawater, the Ni₃S₂/Fe‐NiP_x as OER precatalyst marvelously realizes the ultralow overpotentials of 336 and 351 mV at large current densities of 500 and 1000 mA cm^–2, respectively, with remarkable durability for over 225 h, outperforming most reported advanced OER electrocatalysts. Experimentally, a series of characterization results confirm the reconstruction behavior in the Ni₃S₂/Fe‐NiP_x surface, leading to the in situ formation of Ni(OH)₂/Ni(Fe)OOH with abundant oxygen vacancies and grain boundaries, which constructs the Ni₃S₂/Fe‐NiP_x reconstruction system responsible for the remarkable OER catalytic activity. Theoretical calculation results further verify the enhanced OER activity for Ni₃S₂/Fe‐NiP_x reconstruction system, and unveil that the Fe‐Ni₂P/FeOOH as active origin contributes to the central OER activity.

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