The development of eco-friendly and light-weighted solid-state electrolytes with a wide electrochemical window is critical to apply flexible energy storage devices for powering wearable and implantable electronics. Herein, we report the first demonstration of a quasi solid-state hydrogel electrolyte leveraging the formation of “interfacial wetting water” for facile two-dimensional ion transports instead of typical diffusion in bulk-like water. The hydrogel electrolyte exhibits a wide electrochemical window (2.5 V) in electrical double layer capacitance (EDLC) cell with an extremely effective low salt concentration (6.8 m), 3.1 times lower than the well-known water-in-salt electrolyte (WiSE, 21 m), and a small water retention (24 wt%). Besides, a significant toughness (ultimate tensile strength of 420 kPa and elongation of 6,000%) is achieved. The flexible supercapacitor demonstrates the high specific energy density of 39.1 Wh•kg−1 at 0.5 A•g−1 and 7.6 Wh•kg−1 at power density of 6218 W•kg−1, as well as a subdued self-discharge profile. This interfacial water dominated hydrogel electrolyte provides new directions in designing high-voltage hydrogel electrolyte for safe and sustainable soft energy storage devices.