Abstract: The disclosure provides a novel positive electrode active material for an aqueous potassium ion battery, and a novel potassium ion secondary battery. The positive electrode active material for an aqueous potassium ion battery is represented by the general formula LixMn2O4, where 0<x<2. The aqueous potassium ion secondary battery comprises a positive electrode active material represented by the general formula LixMn2O4 where 0<x<2, and an aqueous electrolyte solution, wherein the aqueous electrolyte solution has a pH of 7.0 to 13.0 and comprises an aqueous solvent and a potassium salt dissolved in the aqueous solvent. The potassium salt is preferably potassium pyrophosphate.