Abstract : Regarding the issue of green chemistry and the vision of human sustainability, a novel  corrosion inhibitor for API 5L carbon steel was explored from Eucheuma seaweed algae. The inhibiting performance of Eucheuma extract was studied by electrochemical measurements such as potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). In this work, the Fourier Transform Infra-Red (FTIR) was also employed to confirm the main phenolic compounds of Eucheuma extract. The electrochemical measurement result indicated that the Eucheuma extract was an efficient corrosion inhibitor in reducing corrosion attacks of API 5L carbon steel in hydrochloric acid medium. The improved corrosion resistance is attributed to the complicated items, including extract concentration and holding time. Eucheuma inhibitor efficiency up to 90% (96.4%) with concentration 500ppm and 30min holding time indicated that Eucheuma could be used as a green inhibitor. It was found that the Eucheuma extract was a mixed-type corrosion inhibitor that inhibit both the cathodic and anodic corrosion reaction. This study was helpful to discover the seaweed algae that was abundant in Indonesia’s marine for inhibiting corrosion of API 5L carbon steel in an aggressive environment.