High-quality poly(3-thiophenemalonic acid) (P3TMA), a water-soluble polythiophene derivative, was successfully electrosynthesized in boron trifluoride diethyl etherate + 50% (by volume) trifluoroacetic acid at lower potential (0.1 V versus Pt). The carboxyl groups make P3TMA highly soluble in water, facilitating its potential application as a blue-light-emitting material. P3TMA film with conductivity of 16 S cm−1 obtained from this medium showed better redox activity and thermal stability. The structure and morphology of the polymer were studied by ultraviolet–visible, Fourier-transform infrared, and nuclear magnetic resonance spectroscopy and scanning electron microscopy, respectively. To the best of our knowledge, this is the first report on electrosynthesis of P3TMA film.