Tannin was successfully extracted from Gelam bark using acetone as the solvent as natural alternatives. The extracted tannin was then used as corrosion inhibitor for mild steel under acidic medium. The gravimetric and electrochemical potentiodynamic corrosion tests were executed at different purified and unpurified tannin concentrations (200-800 ppm) to test the ability to inhibit mild steel corrosion. The results showed that the corrosion rate decreased as tannin concentration increased while the inhibition efficiency increased. The isotherm adsorption found that the Langmuir model was the best model to represent the interaction of tannin inhibitor and the active sites on mild steel surface. The SEM analysis showed that the mild steel morphology changed after the addition of tannin. The presence of blue-black color on the mild steel surface indicated the formation of ferric tannate to protect the surface of mild steel. In conclusion, purified tannin was a better inhibitor compared to unpurified tannin on mild steel in 1 M HCl. � 2018, Malaysian Society of Analytical Sciences. All rights reserved.