Gel polymer electrolytes (GPEs) have received a great deal of attention for use in solid-state supercapacitors (SSCs). However, a majority of the reported GPEs, such as petroleum-derived poly(vinyl alcohol), suffer from flammability, poor water retention, and low ionic conductivity, resulting in poor safety and low capacitance. Herein, we report a high-performance flame-retardant GPE (FRGPE) for SSCs using natural and sustainable marine biomass alginate as a precursor. The obtained lithium alginate/C2H3LiO2 (Li-Alg/LiOAc) FRGPE not only offers excellent flame-retardant performance (high oxygen index value, 35%) but also can effectively retain water to avoid swelling behavior at high temperatures. Therefore, it can completely resolve the safety problems of SSCs. Importantly, the seaweed GPE displays a considerably high ionic conductivity (32.6 mS cm(-1)) because of the amorphous structure and abundant oxygen of the polymer. Accordingly, the safe SSC fabricated by FRGPE with activated carbon electrodes delivers a high specific capacitance, excellent rate performance, and superior stability. The natural and sustainable seaweed GPEs could be promising electrolytes for developing safe and high-performance SSCs.