AbstractSupercapacitors are crucial in renewable energy integration, satellite power systems, and rapid power delivery applications for mitigating voltage fluctuations and storing excess energy. Aqueous electrolytes offer a promising solution for low‐cost and safe supercapacitors. However, they still face limitations in cycle life and wide‐temperature range performance. Here, we present a symmetric supercapacitor utilizing activated carbon electrodes and a “water‐in‐salt” electrolyte (WiSE) based on lithium perchlorate. The WiSE electrolyte exhibits an expanded electrochemical stability window, endowing the aqueous supercapacitor with remarkable stability and long cycle life of over 100,000 cycles at 500 mA g−1 with more than 91 % capacity retention. Moreover, the supercapacitor demonstrates good rate capability and wide temperature operability ranging from −20 to 80 °C. The use of high concentrations of salt in the aqueous electrolyte contributes not only to the enhancement of supercapacitor performance and cycle life but also to the temperature stability range, enabling all‐season operability.