Hydrogen energy carrier produced by water electrolysis in alkaline electrolytes is rather meaningful and significant for global sustainability imperatives, while the high-pH condition usually leads to a poor reversibility of proton adsorption and desorption that significantly determines the hydrogen-generation activity in hydrogen evolution reaction (HER) process. Herein, we demonstrate a remarkable Ru–Mo solid–solution nanocrystal catalyst in alkaline HER process by a very simple but feasible pyrolysis and alkali leaching strategy. Benefiting from the pinning effect and local chemical- and electronic-structure regulations of Mo solute atoms, an ultra-low overpotential (17.3 mV) and an exceptional stability (> 100 h) at the typical current density of 10 mA·cm⁻² are achieved on the ultrasmall Ru–Mo solid–solution nanocrystal catalyst in 1.0 M KOH electrolyte. Density function theory (DFT) calculations gain an insight into the synergistic effect of neighboring Ru and Mo sites in alkaline HER process, where Mo solute atoms are beneficial for the adsorption and activation of water molecules for proton generation and accumulation due to their rich outermost 4d vacant orbitals, while the energy-favorable Ru sites are responsible for the fast deprotonation kinetics of hydrogen intermediates. Our work may provide an interesting route for the development of efficient and stable solid–solution alloy nanocrystals towards alkaline water electrolysis and beyond.