Designing high-performance electrocatalysts toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is essential to reduce the activation barrier and optimize free adsorption energy of reactive intermediates. Herein, we report that incorporating high-valence Cr into NiSe₂ (Cr_xNi_1−xSe₂) kinetically and thermodynamically expedites elementary steps of both HER and OER. The as-prepared Cr_0.05Ni_0.95Se₂ catalyst displays excellent HER and OER activities, with low overpotentials of 89 and 272 mV at the current density of 10 mA·cm⁻² (j₁₀), respectively, and remains stable during operation for 30 h. A low cell voltage of only 1.59 V is required to drive j₁₀ in alkaline media. In situ Raman spectroscopy reveals that Cr incorporation facilitates the formation of NiOOH active species during the OER process. Meanwhile, theoretical explorations demonstrate that high-valence Cr incorporation efficiently accelerates water dissociation kinetics and improves H* adsorption during HER process, lowering the activation barrier of OER and optimizing the adsorption energy of oxygen-based intermediate, thus kinetically and thermodynamically enhancing the intrinsic performance of NiSe₂ for over water splitting. This strategy provides a new horizon to design transition metal based electrocatalysts in the clean energy field.