Exploring highly efficient and low‐cost electrocatalysts for electrochemical water splitting is of importance for the conversion of intermediate energy. Herein, the synthesis of dual‐cation (Fe, Co)‐incorporated NiSe2 nanosheets (Fe, Co‐NiSe2) and systematical investigation of their electrocatalytic performance for water splitting as a function of the composition are reported. The dual‐cation incorporation can distort the lattice and induce stronger electronic interaction, leading to increased active site exposure and optimized adsorption energy of reaction intermediates compared to single‐cation‐doped or pure NiSe2. As a result, the obtained Fe0.09Co0.13‐NiSe2 porous nanosheet electrode shows an optimized catalytic activity with a low overpotential of 251 mV for oxygen evolution reaction and 92 mV for hydrogen evolution reaction (both at 10 mA cm−2 in 1 m KOH). When used as bifunctional electrodes for overall water splitting, the current density of 10 mA cm−2 is achieved at a low cell voltage of 1.52 V. This work highlights the importance of dual‐cation doping in enhancing the electrocatalyst performance of transition metal dichalcogenides. Dual‐cation incorporation makes NiSe2 nanosheet a more effective catalyst. Introducing both Fe and Co atoms with an optimal ratio into porous NiSe2 nanosheets causes evident lattice distortion and strong electronic interaction, leading to a more efficient bifunctionality in overall electrochemical water splitting.