Under the effect of acidic exposure, stainless steel structures of industrial equipment are constantly losing their corrosion resistance, and consequently leading to extensive damage. With the aim of protecting stainless steel equipment from the general corrosion in sulfuric acid, we investigate the inhibitory effect of Dittrichia viscosa extract and the main isolated sesquiterpenes. Potentiodynamic polarization and AC electrochemical impedance are used to demonstrate the efficiency of the tested inhibitors. For a better understanding of the molecular properties and the inhibitory potential of each sesquiterpene, quantum chemical calculations are performed using a hybrid method of B3LYP function with 6-31G (d, p) basis set. The present study provides comprehensive results proving the inhibitory effect of sesquiterpenes on the corrosion of stainless steel in sulfuric acid. The inhibition efficiency of tomentosin attains 95% at 7.5 × 10−5 mol/L. The adsorption data of tomentosin fit well the Langmuir isotherm, while the thermodynamic study substantiates its chemical adsorption. Quantum chemical calculations suggest the reactive sites of interaction and corroborate the experimental findings. The present work might help to understand the corrosion behaviour of stainless steel in sulfuric acid and the inhibitory potential of different sesquiterpenes.