Thermoelectric (TE) materials are important for the sustainable development because they enable the direct harvesting of low-quality heat into electricity. Among them, conducting polymers have attracted great attention arising from their advantages, such as flexibility, nontoxicity, easy availability, and intrinsically low thermal conductivity. In this work, a novel and facile method is reported to significantly enhance the TE property of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films through sequential post-treatments with common acids and bases. Compared with the as-prepared PEDOT:PSS, both the Seebeck coefficients and electrical conductivities can be remarkably enhanced after the treatments. The oxidation level, which significantly impacts the TE property of the PEDOT:PSS films, can also be well tuned by controlling the experimental conditions during the base treatment. The optimal PEDOT:PSS films can have a Seebeck coefficient of 39.2 mu V K-1 and a conductivity of 2170 S cm(-1) at room temperature, and the corresponding power factor is 334 mu W (m(-1) K-2). The enhancement in the TE properties is attributed to the synergetic effect of high charge mobility by the acid treatment and the optimal oxidation level tuned by the base treatment.