Organic thermoelectrics have attracted considerable attention owing to their remarkable advantages, including room-temperature power generation, skin-attachable/wearable applications with biocompatibility, and solution-based high-throughput fabrication. Self-doped conjugated polyelectrolytes (CPEs) constitute a promising class of conductive organic materials that are considered potential candidates for organic thermoelectrics. However, the low power factor of CPEs derived from their low electrical conductivity (σ) has been a major drawback in CPE-based thermoelectrics. Herein, we report a strategy for enhancing the thermoelectric performance of CPEs through a post-treatment using aq. H2SO4 solution. The post-treatment increases σ by two orders of magnitude, originating from H2SO4-induced doping accompanying a significant increase in charge carrier concentration. Consequently, a power factor of 3.0 W m‒1 K‒2 is achieved at room temperature. Furthermore, using this highly conductive H2SO4-doped CPE, we developed flexible thermoelectric generators that allow durable power generation under repetitive mechanical bending stresses. Our findings provide insight into developing high-performance and versatile CPEs for next-generation organic thermoelectrics.