The inhibitive action of Clematis gouriana (CG) on mild steel (MS) corrosion in 1.0 M HCl solution was studied. Inhibition efficiency of CG was carried out by using various weight loss methods, potentiodynamic polarisation, and electrochemical impedance spectroscopy. Inhibition efficiencies of up to 95.70% for CG can be obtained. Adsorption of CG on the MS surface was found to obey the Langmuir adsorption isotherm. Free energy of adsorption calculated from the temperature studies revealed the possibility of both chemisorptions and physisorption. The adsorbed film on the MS surface containing the CG inhibitor was also characterized by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscope and energy-dispersive spectrum. The possible active ingredient responsible for the anticorrosion effect is identified as aporphine alkaloid which is isolated and screened for the anticorrosion effect using electrochemical studies. The possible mode of corrosion inhibition of aporphine alkaloid is also derived using FTIR studies.