Designing high‐performance and cost‐effective electrocatalysts for water splitting at high current density is pivotal for practical industrial applications. Herein, it is found that atomic‐level surface engineering of self‐supported nickel phosphide (NiP) nanoarrays via a facile cation‐exchange method can substantially regulate the chemical and physical properties of catalysts by introducing Co atoms. Such surface‐engineered NixCo1–xP endows several aspects of merits: i) rough nanosheet array electrode structure accessible to diffusion of electrolytes and release of gas bubbles, ii) enriched P vacancies companied by Co doping and thus increased active sites, and iii) the synergy of Ni5P4 and NiP2 beneficial to catalytic activity enhancement. By virtue of finely controlling the Co contents, the optimal Ni0.96Co0.04P electrode achieves remarkable bifunctional electrocatalytic performance for overall water splitting at a large current density of 1000 mA cm−2, showing overpotentials of 249.7 mV for hydrogen evolution reaction and 281.7 mV for oxygen evolution reaction. Furthermore, the Ni0.96Co0.04P electrode at 500 mA cm−2 exhibits an ultralow potential (1.71 V) and ultralong durability (500 h) for overall water splitting. This study implies that the atomic‐level surface engineering of the electrode materials offers a viable route for gaining high‐performance catalysts for water splitting at large current density. The self‐supported NiP nanosheet array electrode with enriched P vacancies and atomic‐level Co doping is achieved by a controlled cation‐exchange strategy excluding complicated chemical reactions and post‐treatment steps. Such NiP nanoarray electrode exhibits excellent performance and long‐term stability (over 500 h) for electrocatalytic water splitting at large current density, outperforming noble‐metal catalysts in electrocatalytic hydrogen evolution reaction and oxygen evolution reaction.