Sulfonated chitosan (S-CTS) doped polyaniline (PANI) was synthesized by template-free method successfully, and it is reported as corrosion inhibitor for the first time. The corrosion inhibition property of PANI was demonstrated for mild steel in acidic environment using weight loss test, Tafel polarization, electrochemical impedance spectroscopy (EIS), scanning electronic microscope (SEM) and adsorption isotherm.