Electrocatalytic water splitting is one of the most desirable ways to produce hydrogen, which is a clean and sustainable energy source. Improving the formation and the exposure of active sites is a key issue in the design of cost-effective and efficient metal-based electrocatalysts, especially for noble metals, in order to cut down the metal content and improve the mass activity of the metals, while maintaining high catalytic activity. Herein, we report a high-performance Ru-based electrocatalyst consisting of Ru single atoms and Ru nanoclusters encapsulated in highly porous N-doped carbon with abundant hierarchical pores (Ru/p-NC). Remarkably, Ru/p-NC shows excellent HER activity in 1.0 M KOH with a low overpotential of 10 mV at 10 mA cm −2 , a small Tafel slope of 17 mV dec −1 as well as good durability for 24 h, outperforming commercial Pt/C and Ru/C catalysts. More importantly, the mass activity and turnover frequency (based on Ru loading) of Ru/p-NC are ultrahigh owing to the low loading of Ru, which are 17 A mg Ru −1 and 8.9 H 2 s −1 , respectively, at a very low overpotential of 25 mV. Experimental results indicate that the porous N-doped carbon support not only leads to exposed active sites but also interacts strongly with the Ru moieties. In addition, it tunes the electronic structure of the catalyst, boosts the stability as well as facilitates mass transport and charge transfer kinetics. This work provides a new route for the synthesis of metal-based porous N-doped carbon hybrid electrocatalysts with abundant exposed active sites and strong metal-support interaction for energy conversion. Ruthenium single atoms and ruthenium nanoclusters dispersed in hierarchically porous N-doped carbon significantly increase the Ru atom efficiency towards excellent electrocatalytic HER activity in alkaline media.