Designing nanocatalysts with fast water dissociation kinetics is key to achieving the industrial current density of the cathodic hydrogen evolution reaction (HER) for green hydrogen production via water electrolysis, which remains a grand challenge. Here, leverage single‐atom engineering,a Ce single‐atom‐doped amorphous RuO _x nanosheet featuring abundant amorphous/crystalline nanojunctions, namely, ac‐Ce _SA RuO _x is successfully developed. When employed as a catalyst for alkaline HER, the ac‐Ce _SA RuO _x catalyst displays an overpotential of only 8.3 mV at 10 mA cm ⁻² and Tafel slope as low as 23.1 mV dec ⁻¹ , outperforming the Ru‐based catalysts reported in recent studies. Anion exchange‐membrane water electrolyzers (AEMWEs) tests using ac‐Ce _SA RuO _x as the cathode catalyst exhibit a low cell voltage of 1.71 V at 1000 mA cm ⁻² , and the performance can be well maintained after 300 h. In situ spectroscopic analyses demonstrate the increased coverage of OH species on ac‐Ce _SA RuO _x for enhanced water dissociation. Density function theory calculations suggest that the Ce‐doped crystalline nanodomains in the amorphous/nanocrystalline nanojunction could effectively reduce the energy barrier to water dissociation, while the Ce‐doped amorphous constituents regulate the H ^* adsorption, which collectively boost the HER performance. This work offers a new strategy to create amorphous/nanocrystalline nanojunctions in nanocatalysts for enhanced electrocatalysis.