Self-acid-doped conjugated polymers (CPs) are a compelling class of materials that add the benefits of a dopant built directly into the polymer structure to the existing advantages inherent in mixed ionic-electronic conductivity.(1) This unique combination makes them ideal candidates for bioelectronic applications that interface directly with physiological environments where traditional dopants might leach out or degrade.(2) The recent rise in interest in organic bioelectronics has seen a corresponding increase in efforts to expand the repertoire of self-acid-doped CPs, which to date have largely been focused towards sulfonate as the dopant group.(3) We present here the synthesis of a poly(3,4-ethylenedioxythiophene) derivative (PEDOT-Th) bearing an ionised thiosulfate group positioned one methylene away from the backbone.(4) The polymer exhibited high water solubility (18.3 mg mL−1), and self-cross-linked under thermal treatment, resulting in water stable films without additives. The films were self-acid-doped, evidenced by a polaron peak in the UV-Vis spectrum (~800 nm) in the absence of an external dopant, and had a conductivity of 3.1 ± 1.7 S cm−1 and an ionisation potential of 4.42 eV. The films could be reversibly switched between redox states under applied voltages, however reverted back to the doped state after the bias was lifted. Motivated by the resilience of the self-acid-doping we are currently leveraging established electrochemical techniques to obtain greater insight into the complex ion transport mechanisms and electronic behaviour of these materials. Crucially, we demonstrate here the first instance of utilizing a thiosulfate functional group to achieve self-acid-doping in a CP and in doing so extend the limited library of self-acid-doped CPs.