Potassium-ion batteries (K-ion batteries; KIBs) are attractive as high-voltage, low-cost energy storage devices. Ionic liquids (ILs) are potential candidates as safe and high-performance electrolytes for large-scale devices. Imidazolium-based ILs are known to possess high ionic conductivities and moderate electrochemical stabilities. In this study, we report the physicochemical and electrochemical properties of K[FSA]- [C(2)C(1)im][FSA] (FSA = bis(fluorosulfonyl)amide; C(2)C(1)im = 1-ethyl-3-methylimidazolium) ILs as new electrolytes for KIBs. A phase diagram constructed from differential scanning calorimetry results indicates that the melting point of this IL is below 273 K at compositions of x(K[FSA]) = 0-0.20 (x(K[FSA]) = molar fraction of K[FSA]). The viscosity, ionic conductivity, and density of the IL were measured for x(K[FSA]) = 0-0.20. The ionic conductivity at x(K[FSA]) = 0.20 is 10.1 mS cm(-1) at 298 K, which is comparable to that of typical organic solvent-based KIB electrolytes and higher than that of other ionic liquid electrolytes for KIBs. The electrochemical stability of M[FSA]-[C2C1 im][FSA] (x(M[FSA]) = 0.20; M = K, Na, and Li) ILs was determined by cyclic voltammetry measurements. The K-based IL exhibits the widest electrochemical window of 5.19 V due to the negative redox potential of the K+/K couple. Thus, the K[FSA]-[C2C1 im][FSA] ionic liquid is expected to be a highly conductive KIB electrolyte.