Exploration of efficient and robust catalysts for electrocatalytic water splitting is paramount yet challenging for economical hydrogen production. Here, nanoforest-like heterostructures composed of inner NiMoS4 nanowires and outer Cr-doped Co3S4 nanosheets were grown on nickel foams (Cr–Co3S4/NiMoS4) as highly efficient bifunctional electrocatalysts. As a result, Cr–Co3S4/NiMoS4 heterostructures exhibit low overpotentials of 72 mV and 243 mV for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) at 10 mA cm−2, respectively. Moreover, the water electrolyzer assembled by Cr–Co3S4/NiMoS4 as bifunctional electrodes reaches 10 mA cm−2 at 1.587 V and maintains exceptional stability over 200 h. The experimental and theoretical characterizations collectively unveil that the charge redistribution occurs at the heterointerface between Cr-doped Co3S4 and NiMoS4, resulting in the regulation of both their electronic structures, which optimizes the adsorption of HER intermediates and decreases the energy barrier of determining step for OER. Additionally, the Cr doping and nanoforest-like morphology increase the intrinsic conductivity and the exposure of active sites, collectively improving the water electrolysis efficiency. This finding presents a promising way to construct and adjust the heterojunction engineering for bifunctional electrocatalysts toward water electrolysis.