In the work, we developed an aqueous symmetric Ru–Cr binary oxide (Ru0.83Cr0.17O2·nH2O)-based electrochemical capacitor (EC) with unwonted high operating voltage in 1M KOH aqueous solution. Electrochemical performance of the symmetric EC was investigated by current-potential response and galvanostatic charge–discharge techniques. Electrochemical data demonstrated that the symmetric EC with a specific capacitance (SC) of ca. 148Fg−1 surprisingly can operate in a large electrochemical window of 1.4V in 1M KOH electrolyte, ensuring that the symmetric EC delivered an energy density of ca. 40Whkg−1. Moreover, the extended cycle life and good power property were obtained meanwhile, indicating that the Ru0.83Cr0.17O2·nH2O-based EC is a promising candidate for aqueous symmetric devices with high energy density and desirable power property.