Partially hydrous RuO₂ nanocluster embedded in a carbon matrix (x-RuO₂@C with x = hydration degree = 0.27) is presented as a bifunctional catalyst for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) for water splitting. Symmetric water electrolyzers based on 0.27-RuO₂@C for both electrodes showed smaller potential gaps between HER and OER at pH 0, pH 14 and even pH 7 than conventional asymmetric electrolyzers based on two different catalysts (Pt/C || Ir/C). Moreover, 0.27-RuO₂@C showed another bifunctional electroactivity for fuel cell electrochemistry involving hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) that are the backward reactions of HER and OER respectively. Pt-level HOR electroactivity was obtained from 0.27-RuO₂@C, while its ORR activity was inferior to that of Pt with 200 mV higher overpotential required. The tetra-functionality of 0.27-RuO₂@C showed the possibility of realizing single-catalyst regenerative fuel cells.