We reported a new and environmentally friendly phloroglucinol additive incorporated polyvinyl alcohol-LiClO4 gel polymer electrolyte for flexible symmetrical solid-state supercapacitors. The incorporation of 10 wt% phloroglucinol into PVA-LiClO4 gel polymer electrolytes results in much higher specific capacitance (114 F g-1) than that without phloroglucinol (60 F g-1), an increase of 90%, because of the redox reaction of phloroglucinol in the system. We used the PVA-LiClO4 with 10 wt% phloroglucinol as a gel polymer electrolyte in combination with nitrogen-doped graphene nanosheet-based electrodes as the anode and cathode to fabricate a symmetrical solid-state supercapacitor. The fabricated supercapacitor involving the phloroglucinol redox additive has an energy density of 2.3 W h kg-1 at a power density of 150 W kg-1, which is higher than the one without a redox additive (1.9 W h kg-1 at a power density of 150 W kg-1). Moreover, the supercapacitor involving the redox additive has good stability; its capacitance remains at up to ∼94% after 5000 charge/discharge cycles.