Endocarp and pericarp extracts of Cereus fricii cactus were used to inhibit carbon steel corrosion in HCl and NaCl solutions by gravimetric assays. Extracts were mechanically obtained, concentrations were fixed and characterized by phytochemical testing and FTIR analysis, and the nature of inhibition was determined by calculation of Gibbs Free Energy. It was established that the extracts have the presence of tannins, saponins, and antioxidant compounds, which was corroborated by the FTIR analysis, showing peaks of absorbance corresponding to groups OH and COOH, with the endocarp extract presenting the highest proportion. Endocarp extracts with 5% concentration showed better performance in inhibiting corrosion than extracts obtained from epicarp, with percentages of 97.71 when inhibiting HCl and 73.91% with NaCl. The presence of tannins, saponins, and antioxidant compounds in extracts of the endocarp and epicarp favour inhibition of metal corrosion.