High energy density and the resolution of safety concerns are of great importance for various energy storage devices. However, conventional lithium-ion batteries and supercapacitors comprise of liquid organic electrolytes may catch fire, or even explode, when exposed to undesirable conditions such as high temperatures. Herein, we describe the synthesis of a high-performance fire-retardant organic gel polymer electrolyte via a facile epoxy open-ring polymerization strategy, by using a functional poly(ethylene glycol) copolymer cross-linking a novel flame retardant, with a brominated epoxy resin, tetrabromo bisphenol A, as the matrix, and swelled using an organic liquid electrolyte (lithium salts in propylene carbonate solvents). The poly(ethylene glycol) section of this fire-retardant gel polymer electrolyte guarantees high ionic conductivity, which is recorded to be as high as 1.09 × 10−3 S cm−1; this is comparable to values obtained for organic liquid electrolytes and superior to the ionic conductivity of previously reported organic gel polymer electrolytes. Furthermore, owing to the polymerization of the flame retardant tetrabromo bisphenol A onto the polymer electrolyte matrix framework, rather than direct dissolution of tetrabromo bisphenol A into the electrolyte, our gel polymer electrolyte exhibits extremely safe fire-retardant properties without negative effects on electrochemical performance. Benefiting from this fire-retardant organic gel polymer electrolyte, the fabricated solid-state supercapacitor exhibits a good electrochemical performance, that is, a high specific capacitance of 166.69 F g−1 at 0.5 A g−1, a large energy density of 42.19 Wh kg−1, and a remarkably flexible performance under bending conditions, which are superior to those for a supercapacitor with a corresponding liquid organic electrolyte. More importantly, the obtained fire-retardant organic gel polymer electrolytes may be useful for the design of advanced next-generation flexible energy devices.