Fish products are highly perishable due to the rapid growth of microorganisms that are present either naturally or through contamination. Synthetic preservatives are commonly used to extend shelf life and ensure quality and safety. However, the increasing consumer preference for natural preservatives and concerns over synthetic ones have driven the food industry to seek natural alternatives. Chestnut by-products, rich in antioxidant compounds such as hydrolysable tannins, phenolic acids, and flavonoids, offer potential as natural food additives to enhance nutritional value and prevent oxidation. This study aimed to assess the potential of phenolic-rich extract from male chestnut flowers (Castanea sativa Mill.) as a natural antioxidant preservative in Atlantic Bonito (Sarda sarda) fish burgers. The experiment evaluated the shelf life of fish burgers stored at 4 °C over ten days. The following four fish burger formulations were prepared: a negative control (no added preservative), a positive control with 0.02% butylated bydroxytoluene (BHT), and two test groups with 0.25% and 0.5% chestnut flower extract (CFE). The initial pH levels of all formulations were around 5.90. By day 10, the pH of the negative control group had slightly decreased to 5.83, while the pH levels in the positive control and CFE groups remained stable, showing no significant differences compared to the initial pH. The thiobarbituric acid reactive substances (TBARS) assay indicated a significant increase in lipid oxidation in the negative control group, reaching 6.80 mg malonaldehyde/kg by day 10. Both concentrations of the CFE effectively reduced the TBARS levels, demonstrating that their antioxidant capabilities are linked to the phenolic composition of the chestnut flowers (1.73 and 2.08 mg MDA/kg by day 10 for 0.25% and 0.5% CFE, respectively). These results suggest that CFE could be an effective natural preservative, improving the shelf life and quality of fish burgers for a duration of 10 days in a refrigerator. Future research should explore the mechanisms of CFE's antioxidant effects and conduct a sensory evaluation to determine the organoleptic properties of the fish burger formulations.