Hydrous ruthenium–tin oxides (denoted as (Ru–Sn)O2·nH2O) were synthesized under a mild hydrothermal condition. The mean particles size of pristine RuO2·nH2O, smaller than 3nm, was decreased with the introduction of Sn into the precursor solutions while it is not significantly affected by varying the Sn content. The textual characteristics of (Ru–Sn)O2·nH2O were analyzed through means of X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), electron diffraction, X-ray photoelectron spectroscopy (XPS), and thermogravimetric analyses (TGA). From the ideal capacitive behavior of pristine oxides with the Ru content ≥40at.%, hydrothermal synthesis favored the formation of hydrous oxides with a novel structure providing excellent pathways for electron hopping and proton diffusion/exchange during the charge storage/delivery process. The introduction of tin oxide was demonstrated to successfully promote the utilization of oxyruthenium species, reaching a maximum C S,Ru of ca. 830F/g for pristine Ru0.6Sn0.4O2·nH2O (measured at 25mV/s).