Extensive efforts have been devoted to the qualification of plant extracts as green corrosion inhibitors for industrial metals, but studies that demonstrate the active component(s) of these extracts remain scarce. We report here that piperine, the major pungent component of peppers, has the best corrosion inhibitive efficiency for copper in HCl among four analogous amide alkaloids isolated from a traditional Chinese medicine. This compound inhibited HCl corrosion more efficiently than cysteine, and did not exhibit markedly decreased efficiency under several harsh experimental conditions. Electrochemical and microscopic analyses suggested that piperine could form a protective layer on the metal surface via both physisorption and chemisorption, reducing the corrosion rate. The adsorption energies of all the test compounds were calculated using a hybrid density functional theory.