Exploring highly efficient Pt-free catalysts for hydrogen evolution reaction (HER) is of great importance for hydrogen (H₂) production. Herein, a novel HER electrocatalyst having abundant ultra-small (2–3 nm) Ru electronically confined by a B, N co-doped polar carbon surface (Ru/(B-N)-PC) was constructed. The Ru/(B-N)-PC catalyst exhibits a low overpotential of 15 mV at the current density of 10 mA·cm⁻², a low Tafel slope of 22.6 mV·dec⁻¹, and superior durability, which outperforms the benchmark Pt/C catalyst. Both experimental characterizations and theory calculations suggest that an electron communication established between B, N co-doped carbon surface and ultra-small Ru nanoparticles with electrons transferred from N atoms to Ru and back-transferred from Ru to B atoms, which exerts a moderate electronic modification of Ru. This, in turn, affords a modest H adsorption energy and a lower H₂O dissociation barrier, leading to the high-performance hydrogen evolution reaction. The work provides meaningful insight into the size control and electronic modulation of Ru catalyst for intrinsic HER activity improvement.