The inhibitor performance of 5-aminoindole (5-AI) on mild steel corrosion in 0.5 M HCl was investigated in relation to the inhibitor concentration using potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and linear polarization (LPR) methods. The effect of immersion time on the corrosion behavior of mild steel was also studied. The impedance data obtained were fitted and modeled using an equivalent circuit model. The results show that 5-AI is an effective inhibitor and has an inhibition efficiency of 90% at 1 × 10−2 M additive concentration. The adsorption behavior of 5-AI was also investigated. For this purpose, the adsorption equilibrium constant (Kads), and the free energy of adsorption (ΔGads) were calculated and discussed. It was found that 5-AI acts as mixed-type inhibitor and obeys Langmuir adsorption isotherm with a free energy of adsorption of −27.71 kJ/mol.