Summary Proton‐exchange membrane water electrolyzers with active and stable non‐precious catalysts are necessary to produce cheap and efficient hydrogen from water electrolysis. In this study, Ni‐Mo‐P ternary alloy catalysts with a wide composition range are fabricated by electrodeposition. Their activity and stability as hydrogen evolution catalysts are evaluated at the level of both half and single cells. We prove that the morphologies, crystalline structures, and electronic status of the electrodeposited catalysts, which play important roles in the catalytic activity and stability, significantly vary with the compositions of the ternary system. The best performing NiMoP catalyst with an optimal composition exhibited a half‐cell performance of 28 mV overpotential at −10 mA cm−2 and a single‐cell performance of 2 Vcell at 1.87 A cm−2, which is superior to the previously reported NiP‐ and MoP‐based electrolyzers. Due to the addition of corrosion‐resistant P, the NiMoP catalyst exhibited exceptional stability of 3 mV h−1 for 48 hours at 1 A cm−2. As a result, the novel ternary Ni‐Mo‐P catalyst prepared by a simple and low‐cost synthesis method is a strongly promising candidate for non‐precious hydrogen evolution catalysts in proton exchange membrane water electrolyzers.