In the present work, the inhibitory properties and the adsorption mechanism of penicillin G for the corrosion of Fe-19Cr stainless steel in an HCl solution were investigated. Electrochemical methods, the open circuit potential (OCP) and the current potential curves (I-V) were used. The inhibition efficiency of the penicillin G was calculated and found to be up to 60% for hydrochloric acid. The adsorption of this inhibitor was described by Langmuir isotherm, and the negative values of Gibbs energy indicate the nature of interactions between the inhibitor molecules and the metal surface.