 Herein, Elaeocarpus seed (ES) extract was studied in 3 M HCl solution as a efficacy corrosion inhibitor for the mild steel through mass loss, electrochemical Tafel diagrams, impedance spectroscopy, atomic absorption spectroscopy, scanning electron microscopy, and EDX techniques. During weight loss technique, it is observed that increasing the concentration enhances the protective role of the plant product. The Tafel diagrams showed a mixed role of the ES extract. In impedance spectroscopy, the area of depressed semicircle has a direct relationship with the plant extract concentration, which clearly demonstrating the inhibition property of ES extract. 93.370, 97.111, 89.015, and 90.528% are the maximum inhibition efficiencies obtained from the weight loss, atomic absorption spectroscopy, Tafel plot, and impedance spectroscopy techniques, respectively. Mathematical and quantum chemical studies further support the experimental results.