Hydrogen evolution reaction (HER) is a vital step for electrochemical energy conversion technologies. However, developing cost-efficient electrocatalysts remains a huge challenge. Here, we report that Cr-doped NiS (Cr-NiS) in situ grown on Ni foam via a hydrothermal sulfurization reaction can be an ultra-highly efficient and low-cost electrocatalyst for HER in alkaline condition. We find that the Cr-doping can trigger the formation of NiS and NiS, leading to the suppression of NiS in the nanostructure. We show that Cr-NiS exhibits an excellent HER activity with an ultralow overpotential of 81 mV at 10 mA cm, better than those of most nickel sulfide catalysts, and good stability. Our combined experimental and computational study reveals that the excellent performance is contributed by: (1) stabilized NiS phase, (2) strongly improved conductivity, (3) greatly enhanced active sites, and (4) reduced Gibbs free energy for hydrogen adsorption (ΔG) due to the Cr-doping. We believe that this work provides a simple approach to design cost-effective and ultra-highly efficient HER electrocatalysts for practical applications.