The objective of this study is to contribute to the promotion of Cistus monspeliensis L (CM) plant as a green inhibitor of mild steel in a hydrochloric acid solution that behaves as a source for ecological guarantee high performance at a cheaper price. The aqueous extract of CM (CMAE) was evaluated by electrochemical method. The solutions attained show that the inhibition efficiency increases with the increase in inhibitor dilution and the variation in the inhibitory efficiency depend mainly on the mixture of organic compound present in CMAE (leaves, flowers, and stems) at 0.2 g/l to reach 88%, 81%, and 70%, respectively. The antioxidant activity was also evaluated using the DPPH method. The propriety of inhibitors was identified by FTIR and UV-visible spectrophotometric methods and the surface of mild steel characterized by the SEM method. The possible reactivity performances of the phenolic and flavonoid contents (DPPH, L-AA, GA, and Q) in CMAE were discussed in the framework of DFT analyses. (C) 2021 Elsevier B.V. All rights reserved.