The aim of this study was to investigate the influence of varying concentrations of Terminalia catappa fruit peel extract on the corrosion rate during immersion periods of 2 days, 4 days, and 6 days for ASTM A36 steel. The ASTM A36 steel specimens were immersed in a corrosive medium containing 2% NaCl, with inhibitor compositions of 0%V, 1%V, 2%V, and 3%V being tested. Measurement results revealed that as the concentration of Terminalia catappa fruit peel extract increased, the corrosion rate proportionally decreased. The lowest corrosion rate was observed in samples with a 3%V inhibitor composition, subjected to a 4-day immersion, measuring at 0.02321 mmpy. Moreover, with the augmentation of fruit peel extract concentration, inhibition efficiency exhibited an augmented trend (83.02% for the 3%V composition, and the lowest at 41.84% for the 1%V composition).