The bifunctional catalysts for the hydrogen and oxygen evolution reactions (HER and OER) with high efficiency, low cost, and easy integration for future renewable energy systems are highly desirable. Here, on the basis of first-principles calculations, we predicted a two-dimensional (2D) metal–organic framework (MOF) bifunctional electrocatalyst, namely, bis(iminothiolato)nickel (NiIT) monolayer, for overall water splitting. The semi-metallic properties and low HER/OER overpotentials (−0.15/0.50 V) ensure the remarkable electrocatalytic performance of the 2D MOF electrocatalyst. The spatially separated HER and OER active sites with different electronegativities facilitate the electrocatalytic processes. Our findings highlight a promising precious-metal-free bifunctional electrocatalyst for efficient overall water splitting, as well as a novel strategy in catalyst design.