The inhibiting action of Piper longum seed extracts was investigated as the corrosion inhibitors of aluminium in 1 M NaOH solution using potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) methods, and weight loss measurements. The results revealed that P. longum extract was an effective inhibitor, and the inhibition efficiencies obtained from polarization and weight loss experiments were in good agreement. Using the potentiodynamic polarization technique, the extract was proving to have a mixed-type character for aluminium in alkaline solution by suppressing both anodic and cathodic reactions on the metal surface. The adsorption isotherm model was determined according to fitted results obtained from the weight loss experiments performed on the aluminium specimen in 1 M NaOH solution at 308 K. Thermodynamic parameters such as activation energy and free energy of adsorption were calculated and given in kJ mg−1.