Objective To investigate the corrosion inhibition of carbon steel in HCl medium by melamine and the mechanism. Methods The electrochemical parameters and inhibitive efficiencies were obtained by measuring the potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) for carbon steel in the solution containing different concentrations of melamine, and then combined with the result of quantum chemical calculations, the inhibition mechanism of melamine was discussed and analyzed. Results Melamine was a mixed-type inhibitor for the corrosion of carbon steel in 1 mol/ L HCl solution, and its inhibitive efficiency increased with increasing inhibitor concentration. The inhibition effect of melamine was produced by the adsorption of melamine molecules onto the carbon steel surface through nitrogen atoms, the Mulliken charges of which were relatively more negative. The adsorption process was a spontaneous process, mainly physical adsorption, and obeyed the Langmuir isotherm equation. Conclusion Melamine could inhibit the corrosion of carbon steel in HCl medium.