An electrolyte solution for a potassium ion battery or a potassium ion capacitor, the electrolyte solution comprising at least one potassium salt compound selected from the group consisting of potassium bis(trifluoromethanesulfonyl)amide and potassium bis(fluorosulfonyl)amide and at least one solvent selected from the group consisting of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, pentaethylene glycol dimethyl ether, ethylene carbonate, and propylene carbonate, in which a concentration of the potassium salt compound in the electrolyte solution is from 1.5 mol/kg to 12.0 mol/kg.