The corrosion inhibition performance of triethanolamine (TEA) with AZ91D magnesium alloy in 3.5 wt% NaCl solution has been studied using an immersion experiment, electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, surface analysis, and quantum chemical methods. The results revealed that TEA was a good corrosion inhibitor. The polarization plots indicated that TEA served as a mixed-type inhibitor. The adsorption of TEA molecules was found to follow the Langmuir adsorption isotherm, which was a spontaneous, exothermic process of increased entropy. TEA could considerably improve the corrosion resistance of the Mg alloy in 3.5 wt% NaCl.