In view of the need for environmental friendliness and cost effectiveness, the enhancement of the energy density of the aqueous supercapacitor is in high demand. Recently, concentrated aqueous electrolytes known as water-in-salt electrolytes (WiSEs) have attracted much attention due to their broad electrochemical stability window (2–3 V) relative to that of conventional dilute aqueous electrolytes (∼1 V). Meanwhile, the development of redox-active electrolytes has provided a great opportunity to improve the capacitance of the supercapacitors by providing an additional pseudocapacitive contribution. Herein, a supercapacitor containing a dual redox-active (RA) WiSE is demonstrated that combines the benefits of the wide voltage window of the WiSE and the high capacitance arising from the RA species, thus significantly amplifying the energy density of the supercapacitor. Moreover, the voltage plateau arising from the simultaneous redox reactions can deliver a constant power output, representing a distinctive and attractive alternative to the conventional aqueous supercapacitors.