Achieving highly efficient hydrogen evolution reaction (HER) in alkaline media is a great challenge. Single-atom catalysts with high-loading amount have attracted great interest due to their remarkable catalytic properties. Herein, by using nickel foam as the substrate, the authors design and precisely synthesize atomic ruthenium (Ru)-loaded nickel hydroxide ultrathin nanoribbons (R-NiRu) with a high atomic Ru loading amount reaching approximate to 7.7 wt% via a one-step hydrothermal method. The presence of concentrated Cl- in the synthetic system is beneficial for constructing ultrathin nanoribbons, which, with abundant edge -OH groups, make it easy to trap atomic Ru. Taking advantage of the synergy between atomic Ru and the nanoribbon morphology of nickel hydroxide, R-NiRu exhibit a low overpotential of 16 mV for HER at 10 mA cm(-2) and a Tafel slope of 40 mV dec(-1) in aqueous 1.0 m KOH solution, which are superior to those of commercial Pt/C (overpotential of 17 mV at 10 mA cm(-2), Tafel slope of 43 mV dec(-1)). Density functional theory (DFT) calculation results demonstrate that atomically dispersed Ru can significantly reduce the HER energy barrier. Moreover, R-NiRu maintains exceptional stability after 5000 cyclic voltammetry cycles. This efficient and facile synthetic strategy provides a new avenue for designing efficient catalysts.