All-temperature flexible supercapacitors have not been realized because of challenges from conventional hydrogel electrolytes. Large amounts of water in hydrogel electrolytes inevitably freeze and restrict ion transport at subzero temperatures, and their structures are unstable under high temperature. Here, all-temperature flexible supercapacitors are reported based on an antifreezing and thermally stable montmorillonite/poly(vinyl alcohol) (MMT/PVA) hydrogel electrolyte. MMT materials enhance the thermal stability of the hydrogel, and their lamellar structures facilitate ion conduction due to formation of oriented conductive pathways. The aqueous electrolyte with a freezing point below -50 °C is employed by simply introducing dimethyl sulfoxide. The electrolyte exhibits high ionic conductivity of 0.17 × 10-4 and 0.76 × 10-4 S cm-1 under -50 and 90 °C, respectively. The supercapacitor delivers high capacities under a wide temperature range from -50 to 90 °C and displays excellent cycling stability over 10000 cycles. Because of the hydrogel electrolyte's superior mechanical properties, the device gives stable energy capacity under flexible conditions.