It is highly desired to design and develop highly efficient and durable electrocatalysts for hydrogen evolution reaction and oxygen evolution reaction under alkaline condition. In this study, three dimensional freestanding porous Cu foam in situ armored CoNi alloy nanosheet arrays with tunable compositions is synthesized via a facile and cost-efficient electrodeposition strategy. With the optimal nanosheet composition (Co81Ni19), highly exposed active surface, improved electrical conductivity, the as-fabricated composite exhibits intriguing properties for efficient overall water splitting in alkaline media. Such freestanding Co81Ni19 alloy nanosheet arrays demonstrate superior electrocatalytic performances with the overpotentials of only 132 and 240 mV for hydrogen evolution and oxygen evolution to reach a catalytic current density of 10 mA cm−2 in 1.0 M KOH. Meanwhile, the Co81Ni19 exhibits high electrochemical stability with almost no detectable activity decay after 36,000 s of continuous operation. Remarkably, this study paves the way to design high-performance and easy-accessible transition metal alloy-based bifunctional catalysts for overall water splitting. Meanwhile, these transition metal-based macroscopic assemblies may be attractive candidates for a wide range of practical energy conversion and storage applications.