<p>Steel is the most important material used in buildings, industries, and households, but it is adversely affected by corrosion. The search for sustainable and eco-friendly corrosion inhibitors has gained significant attention due to environmental concerns associated with synthetic alternatives. In this study, for the first time, <i>Casuarina Equisetifolia</i> seed extract was explored as a novel green corrosion inhibitor for mild steel (MS) in an acidic medium. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) studies confirmed that the extract functions as a mixed-type inhibitor, forming an adsorbed protective layer on the metal surface. The inhibition efficiency (IE) increased with concentration, reaching a maximum of 92.44% at 400 ppm, effectively reducing the corrosion rate from 1.52&#xa0;mg cm^− 2 h^− 1 to 0.69&#xa0;mg cm^− 2 h^− 1. However, at excessively high concentrations, multilayer formation or desorption slightly diminished the protective effect. Contact angle measurements revealed an increase in surface hydrophobicity, further preventing electrolyte interaction with MS. UV-Vis analysis indicated strong interaction between the bioactive compounds in the extract and Fe²⁺/Fe³⁺ ions, confirming the inhibition mechanism. The gradual enhancement in surface smoothness with increasing inhibitor concentration further supported the protective effect. The novelty of this work lies in utilizing the underexplored <i>Casuarina Equisetifolia</i> seed extract as a sustainable corrosion inhibitor, demonstrating excellent efficiency and surface-modifying ability without any toxic or synthetic additives. The study established the extract as an effective, renewable, and eco-friendly alternative to conventional synthetic inhibitors, offering promising applications in corrosion control for industrial settings.</p>