The design of high-performance electrocatalysts for alkaline water splitting is of significant importance for the development of a sustainable hydrogen economy. Herein, heterostructured Co₃Mo nanoparticles/porous CoMoO₃ nanosheets (Co₃Mo/CoMoO₃ NPSs) were constructed for alkaline water splitting by annealing CoMoO_x nanosheets under controlled atmospheres. Thanks to its interfacial electronic structure and increased electrochemical active area, Co₃Mo/CoMoO₃ NPSs exhibit impressive hydrogen evolution reaction (HER) activity with an overpotential of 334 mV at 1000 mA cm^-2 and a Tafel slope of 46.4 mV per decade in 1.0 M KOH, which outperforms Pt/C catalyst (621 mV and 74.7 mV per decade). Density functional theory calculations illustrate the electron transfer from Co₃Mo to CoMoO₃ at the interfaces, where electron accumulation on CoMoO₃ favors the dissociation of H₂O molecule, and electron-deficient Co atoms in Co₃Mo have optimized H* absorption energy for HER. The Co₃Mo/CoMoO₃ NPSs also exhibit higher oxygen evolution reaction activity than the RuO₂ catalyst. Moreover, the water electrolyzer using Co₃Mo/CoMoO₃ NPSs as both cathode and anode only requires 1.59 V to deliver a current density of 100 mA cm^-2 in 1.0 M KOH, which outperforms benchmark Pt/C || RuO₂ electrodes couple with 1.69 V to reach the same current density, providing great potential for large-scale applications.