Pt-based catalysts are used commercially for the hydrogen evolution reaction (HER), even though the low earth abundance and high cost of platinum hinder scale-up applications. Ru metal is a promising alternative catalyst for HER owing to its lower cost but similar metal–hydrogen bond strength to Pt. However, designing an efficient and robust Ru-based electrocatalyst for pH-universal HER is challenging. Herein, we successfully synthesized N-doped carbon (NC) supported ruthenium catalysts with different Ru sizes (single-atoms, nanoclusters and nanoparticles), and then systematically evaluated their performance for HER. Among these catalysts, the Ru nanocluster catalyst (Ru NCs/NC) displayed optimal catalytic performance with overpotentials of only 14, 30, and 32 mV (at 10 mA·cm⁻²) in 1 M KOH, 1 M phosphate buffer saline (PBS), and 0.5 M H₂SO₄, respectively. The corresponding mass activities were 32.2, 12.1 and 8.1 times higher than those of 20 wt.% Pt/C, and also much better than those of the Ru single-atoms (Ru SAs/NC) and Ru nanoparticle (Ru NPs/NC) catalysts, at an overpotential of 100 mV under alkaline, neutral and acidic conditions, respectively. Density functional theory (DFT) calculations revealed that the outstanding HER performance of the Ru NCs/NC catalyst resulted from a strong interaction between the Ru nanoclusters and the N-doped carbon support, which downshifted the d-band center and thus weakened the *H adsorption ability of Ru sites.