Precise regulation on interfacial electronic coupling is essential to achieve efficient catalysts with tailored electrocatalytic behaviors but remains challenging. Herein, a straightforward topochemical strategy is presented to realize Co‐based heterostructured nanofiber stereoassembled with PO coupled nanosheet (CoHF/PO). By constructing well‐defined metal oxide/phosphide interface, prominent electron redistribution can be established via interfacial PO coupling, which is strongly correlated with the catalytic activities. Density functional theory calculations indicate that the rational interface engineering renders accelerated charge transfer and more importantly, optimized surface adsorption/desorption behaviors, contributing to boosted kinetics for both hydrogen evolution reaction and oxygen evolution reaction. Particularly, CoHF/PO featuring promoted intrinsic activity and accessible active sites exhibits intriguing activity and stability at higher current densities in alkaline media, surpassing commercial Pt/C and Ir/C. This study is expected to demonstrate noteworthy promise of covalent coupling toward modulated electronic environment and interface chemistry for electrocatalytic applications and beyond. Co‐based heterostructured nanofiber stereoassembled with P–O coupled nanosheet is constructed via a topochemical strategy. The regulated electronic structures and charge distribution via interfacial P–O covalent coupling are beneficial for the adsorption/desorption of intermediates in the hydrogen evolution reactio and oxygen evolution reaction process, contributing to significantly promoted overall water splitting performance.