Metal sulfide clusters are attractive components for flow batteries owing to the abundance of their constituent atoms and their tunable size, solubility, and redox properties. Here, we demonstrate that we can prepare an atomically precise cobalt sulfide cluster in a single step using low-cost precursors and water solubilizing phosphine ligands. The resulting cluster undergoes two electrochemically reversible oxidations in aqueous solutions and is stable in air. The first redox process is chemically reversible during charge–discharge experiments using a static cell and aqueous solutions of NaCl. An aqueous cell comprising methyl viologen (MV²⁺) as the anolyte and an anion exchange membrane provides an operating V_cell = 0.63 V.