Iron disulfide or pyrite (FeS₂) has emerged as a promising transition metal sulfide-based supercapacitor owing to its abundance and superb electrochemical properties. However, FeS₂ still faces major hurdles in realizing its full potential, such as a low energy density and poor conductivity. In this study, we report a high-performance FeS₂ supercapacitor synthesized by a direct one-step process with the help of polyvinylpyrrolidone (PVP). The incorporation of PVP on the active materials prevented dendritic expansion and acted as a binding for solving the current FeS₂ limitations, while facilitating a one-step synthesis process. Additionally, PVP could enhance the electrochemical performance by enabling faster ion movement. An FeS₂/PVP nanocomposite was successfully synthesized, and used in an asymmetric supercapacitor, demonstrating a high specific capacity of 735 F g⁻¹ (at 2 A g⁻¹) and a high energy density of 69.74 W h kg⁻¹ (at 911 W kg⁻¹). The superior electrochemical properties of FeS₂/PVP were enabled by the lower charge-carrier resistance and better surface passivation by PVP, as demonstrated by both electrochemical experiments and first-principles calculations. The high-performance supercapacitor of FeS₂ presented in this study synthesized in situ by an efficient method provides a new insight into novel supercapacitor electrodes.