The inhibitory action of melatonin drug on the corrosion of carbon steel in 0.5 M H₂SO₄ solution was investigated using galvanostatic polarization, electrochemical impedance spectroscopy, potentiodynamic anodic polarization techniques and density functional theory (DFT). It has been found that as the concentration of melatonin drug increases the values of corrosion current density and the values of double layer capacitance decreases, while the values of charge transfer resistance and the percentage of inhibition efficiency increase. This donates that the inhibiting effect of melatonin drug. Polarization date proves that the melatonin drug acts as mixed inhibitor. Melatonin drug inhibits the pitting corrosion of carbon steel in chloride containing solution by shifting the pitting corrosion to more noble direction. The adsorption of melatonin on the carbon steel surface obeys Langmuir adsorption isotherm. The relevancy of quantum chemical descriptors to the performance of melatonin as corrosion inhibitors was investigated.