A natural antioxidant extract obtained from oil mill wastewater was used for the formulation of basil pesto sauce, with the aim to improve quality and stability during storage. The antioxidant extract was added to traditional ingredients (basil, cheese, oil, etc.) and after preparation, packaging, and thermal treatment it was submitted to storage (monitored for 90 days). Fresh samples were stored at 4 °C and pasteurized samples were stored at room temperature. The effect of natural antioxidant addition on basil pesto sauce was evaluated for the main qualitative attributes, such as: physicochemical, microbiological, and antioxidant parameters. The principal results showed that the addition of a natural phenolic extract led to an evident reduction in pH, attaining food safety values under pH 4. The high oxidative stability observed in the basil pesto sauces fortified with the phenolic extract suggests that the incorporation of phenolic compounds delays the propagation phase of lipid oxidation.