© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Thermocells convert waste heat to electricity without any pollution; however, the high cost and corrosivity of redox species hinder their commercialization. In this work, a thermocell that utilizes abundant polysulfide as redox species was demonstrated for the first time. 1-Butyl-1-methylpyrrolidinium polysulfide [(P14)2S3] was synthesized, and the redox species were prepared by the addition of sulfur to the (P14)2S3 solution in DMSO. In thermoelectric measurements, the Seebeck coefficient changed from −0.68 to +0.5 mV K−1 through addition of sulfur to the cell. Operando UV/Vis spectroscopy and open-circuit voltage analysis revealed that this effect was attributed to the change in the dominating redox reactions by the addition of sulfur. This result also provides a thermodynamic view on polysulfides electrochemistry, which is of high importance for lithium–sulfur batteries.