PEDOT:PSS has found an increasing interest in materials science for its advantages such as transparency, low cost, low environmental impact, conformality and easiness for deposition. However, its poor electrical properties have led the research towards the improvement of its conductivity by several ways, most of them concerning post-deposition treatments. In this work, we propose a novel “in-solution” doping method by the acidification of PEDOT:PSS with sulphuric acid. Polymer films with different acid content were obtained by spin coating and characterized after deposition and aging. Sheet resistances as low as 40 Ohm/sq, with an average transmittance of ≈75% in the visible range, were obtained. Sample doped with this method also showed good aging resistance. Our results have shown that our novel in-solution method can be an efficient and easy strategy for doping PEDOT:PSS.