Corrosion inhibition analysis and the adsorption behaviour of Valeriana wallichii root extract for mild steel in 0.5 M H₂SO₄ solution has been investigated utilizing weight loss, electrochemical impendence spectroscopy (EIS), potentiodynamic polarization, SEM, AFM UV-visible spectroscopy, FT-IR, and quantum chemical calculations. Both the electrochemical and weight loss estimations indicated that the root extract of Valeriana wallichii shows excellent inhibition performance of up to 93.47% for mild steel at 500 mg L⁻¹ concentration in 0.5 M H₂SO₄ solution at 298 K. Naphthoic acid, iridoid and an iridoid analogue are the major phytochemical constituents in the extract of Valeriana wallichii, which possess oxygen atoms (lone pair electrons) and π electrons that facilitate adsorption onto the metal surface. The adsorption of this extract obeys the Langmuir adsorption isotherm. Polarization investigations confirmed that the root extract of Valeriana wallichii acts as a mixed type of inhibitor. The surface studies confirm corrosion retardation in the presence of the extract due to the formation of a protective layer on the surface of mild steel.