friendly and sustainable sources of power are being looked at in order to meet the demand. Among all the developing technologies, solar-light-coupled electrochemical water splitting is widely perceived as one of the best solutions due to the generation of hydrogen -a sustainable and clean source of energy. [1] It is known that noble group compounds, namely Pt and RuO 2 /IrO 2 , are the state-of-the-art electrocatalysts toward two critical half-reactions of water splitting -hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). [2] However, the high price and natural shortage impede their further application. Therefore, developing lowcost, efficient electrocatalyst is urgently desired.Motivated by the challenge, a wide range of materials have been investigated and developed. [3] Among them, nickelbased materials have attracted a great deal of attention mainly because their eight valence electrons in 3d orbitals enable the easy alteration of electronic structure through phase engineering (laser-induced, mechanical strain, weak Ar plasma treatment, e-beam irradiation, and plasmonic hotelectron-induced), heterostructuring (NiS 2 -MoS 2 , Ni 3 S 2 -MoS 2 , V-Ni 2 P). [3c,4] A representative compound in this nickel-based family, pyrite-type nickel sulfides (NiS 2 ) is emerging as a highly Developing highly efficient earth-abundant nickel-based compounds is an important step to realize hydrogen generation from water. Herein, the electronic modulation of the semiconducting NiS 2 by cation doping for advanced water electrolysis is reported. Both theoretical calculations and temperaturedependent resistivity measurements indicate the semiconductor-to-conductor transition of NiS 2 after Cu incorporation. Further calculations also suggest the advantages of Cu dopant to cathodic water electrolysis by bringing Gibbs free energy of H adsorption at both Ni sites and S sites much closer to zero. It is noteworthy that water dissociation on Cu-doped NiS 2 (Cu-NiS 2 ) surface is even more favorable than those on NiS 2 and Pt(111). Thus, the prepared Cu-NiS 2 shows noticeably improved performance toward alkaline hydrogen and oxygen evolution reactions (HER and OER). Specifically, it requires merely 232 mV OER overpotential to drive 10 mA cm −2 ; in parallel with Tafel slopes of 46 mV dec −1 . Regarding HER, an onset overpotential of only 68 mV is achieved. When integrated as both electrodes for water electrolysis, Cu-NiS 2 needs only 1.64 V to drive 10 mA cm −2 , surpassing the state-of-the-art Ir/C-Pt/C couple (1.71 V). This work opens up an avenue to engineer low-cost and earth-abundant catalysts performing on par with the noble-metal-based one for water splitting.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.