Redox flow batteries (RFBs) have been limited by low energy density and high cost. Here, we employ highly-soluble, inexpensive and reversible polysulfide and iodide species to demonstrate a high-energy and low-cost all-liquid polysulfide/iodide redox flow battery (PSIB). In contrast to metal-hybrid or semi-solid approaches that are usually adapted for high-energy RFBs, the all-liquid characteristic of the PSIB is crucial to enable practical scale-up development. Combining the achieved energy density (43.1WhL−1 Catholyte+Anolyte) and the inherent low materials cost of sulfur and iodine compared to vanadium, the PSIB system demonstrates a significantly lower materials cost per kilowatt hour ($85.4kWh−1) compared to the state-of-the-art vanadium-based redox flow batteries ($152.0–154.6kWh−1). Operando UV-Visible spectroscopy reveals superior electrochemical reversibility of both electrolytes. With its demonstrated energy density, inherent low material cost and benign chemical natures, the all-liquid PSIB offers a promising solution for high-energy-density and low-cost energy storage applications.