<p>The strength, longevity, and safety of metal buildings exposed to harsh acidic conditions like HCl and H₂SO₄ are all greatly impacted by corrosion. With potential uses in acid pickling and industrial cleaning procedures, <i>Phanera vahlii</i> extract was examined in this work as a green corrosion inhibitor for mild steel in 0.5&#xa0;M H₂SO₄. Gravimetric tests, UV–visible spectroscopy, scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS), potentiodynamic polarization (PDP), and quantum chemical computations were used to assess the inhibitory performance. The findings showed that at 4000&#xa0;mg L⁻¹, the greatest inhibitory efficiency was 96.47%. The development of a protective adsorbed layer was indicated by the EIS analysis, which revealed a considerable increase in charge transfer resistance with increasing inhibitor concentration. The extract functions as a mixed-type (bifunctional) inhibitor, according to PDP data. Surface protection in the presence of the inhibitor was further corroborated by SEM observations. Phytoconstituents like Quercetin-3-O-rhamnoside and Davinvolunol B, which have heteroatoms and π-electron systems that promote adsorption onto the metal surface, are thought to be responsible for the anticorrosion activity. All things considered, <i>Phanera vahlii</i> extract inhibits corrosion in acidic medium in an effective and eco-friendly manner. To provide qualitative molecular insights into adsorption behaviour, theoretical calculations were carried out utilizing representative phytochemicals reported in the literature.</p> Graphical abstract <p></p>