We investigated the electronic structures of low-molecular-weight dithiobiuret (DTB) compounds in both reduced and oxidized states along with those of a DTB polymer in the discharged and charged states using X-ray absorption near edge structure (XANES) measurements. DTB is a simple and lightweight organosulfur molecule with two neighboring tautomeric thiol/thione groups (–SH/>CS) that can form disulfide (S–S) bonds by reversible oxidative cyclization. The DTB polymer, composed of alternating DTB and phenylene (Ph) units, exhibited a high capacity (about 260 mA h g−1) without any significant degradation in the charge and discharge cycle characteristics. Based on XANES measurements, charge–discharge experiments, and quantum chemical calculations, we propose that the reactions of two types of redox couples contribute the redox reaction of the DTB polymer.