AbstractRu nanoparticles (NPs) and single atoms (SAs)‐based materials have been investigated as alternative electrocatalysts to Pt/C for hydrogen evolution reaction (HER). Exploring the dominant role of atomic‐ and nano‐ruthenium as active sites in acidic and alkaline media is very necessary for optimizing the performance. Herein, an electrocatalyst containing both Ru SAs and NPs anchored on defective carbon (Ru_SA+NP/DC) has been synthesized via a Ru–alginate metal–organic supramolecules conversion method. Ru_SA+NP/DC exhibits low overpotentials of 16.6 and 18.8 mV at 10 mA cm⁻² in acidic and alkaline electrolytes, respectively. Notably, its mass activities are dramatically improved, which are about 1.1 and 2.4 times those of Pt/C at an overpotential of 50 mV in acidic and alkaline media, respectively. Theoretical calculations reveal that Ru SAs own the most appropriate H* adsorption strength and thus, plays a dominant role for HER in acid electrolyte, while Ru NPs facilitate the dissociation of H₂O that is the rate‐determining step in alkaline electrolyte, leading to a remarkable HER activity.