The inhibitory properties of Swietenia macrophylla (SM) leaf extract was tested on the corrosion of zinc in 0.5M HCl solution using gravimetric and potentiodynamic polarization measurements. It was observed that the SM leaf extract acts as a good inhibitor for zinc at all concentrations. It was however found that, the inhibition efficiency (% IE) of SM increased with increase in concentration but decreased with increase in temperature. The mean value of activation energy, Ea for the inhibitor studied (13.52 kJmol^-1 ) is lower than 40.0 kJmol^-1 implying that the inhibitor is physically adsorbed on zinc surface. All the values of heat of adsorption Qads and mean values of free energy change of adsorption (-12.46 and -13.08kJmol^-1 ) are negative indicating that the inhibition process is spontaneous and exothermic.