Modulating the size and dispersion of metal particles is highly desirable in nanocatalysis. Herein highly dispersed ruthenium clusters confined in nitrogen-doped carbon (RuNC) were synthesized by a one-pot pyrolysis of Ru(phen)(3) organometallic compounds and glucose. The size of the Ru clusters was finely controlled at a cluster level (1.7 nm) to expose more catalytic active sites. The interaction between the Ru clusters and N species not only facilitated the uniform dispersion, but also prevented the aggregation of Ru clusters. This cluster-level RuNC manifested extraordinary catalytic performance towards the hydrogen evolution reaction (HER) and was comparable to or even better than a commercial Pt/C catalyst in basic and acidic media, respectively. Moreover, based on density functional theory (DFT) calculations, it was demonstrated that the N dopants in the carbon framework efficiently tailored the charge density distribution of the Ru centres and promoted the adsorption of H* on the catalyst, thus contributing to the high catalytic activity. (C) 2020 Elsevier Inc. All rights reserved.