Over the last decade, the effectiveness of phytochemicals as corrosion inhibitors in acid environments has been confirmed. The study shows that using plants could be a cost-effective and environmentally friendly alternative to synthetic inhibitors that can be expensive and dangerous. In particular, the inhibitory properties and the adsorption behavior of genistein, a chemical compound extracted from the flower extract of Retama monosperma (L.) Boiss, were investigated for their durable application in protecting carbon steel in 1 M HCl medium. The results showed that, genistein had an exceptional performance in inhibiting corrosion with a significant inhibition efficiency of 78% at 5 × 10−4 M. Furthermore, scanning electron microscopy revealed that a protective layer had formed on the surface of the carbon steel, helping to improve corrosion resistance. A study on the effects of temperature revealed that the inhibitory efficacy of genistein decreased with increasing temperature. To further analyze the adsorption mechanism, a theoretical analysis based on density functional theory (DFT) method and molecular dynamic (MD) simulations was carried out, which demonstrated the interaction of the adsorbed molecules with the steel surface.