The spread of Westernized dietary patterns, characterized by high intake of sugar-sweetened beverages, processed foods, red and processed meat, sodium, unhealthy fats, and refined grains, represents an important aspect of nutrition transition and is associated with increased risk of noncommunicable diseases and metabolic syndrome. This policy brief is based on two complementary studies: first, the development of the Westernized Diet Index (WDI), which used evidence from the scientific literature to assign coefficients to 30 food groups or dietary components according to their associations with metabolic syndrome and its components; and second, the validation of the WDI among adults participating in the Fasa cohort study. The findings show that sugar-sweetened beverages, processed foods, red meat, sodium, and hydrogenated fats are the most unfavorable components of Westernized diets, while dietary fiber, plant-based metabolites, vitamins and minerals, nuts and seeds, and fish have the strongest protective coefficients. In the Fasa cohort, higher WDI scores, indicating lower adherence to Westernized dietary patterns, were associated with lower odds of metabolic syndrome and more favorable metabolic biomarkers, including waist circumference, BMI, fasting blood glucose, triglycerides, and blood pressure. Based on these findings, the WDI can be used as a nutritional monitoring and research tool to assess dietary Westernization, identify unhealthy dietary patterns, and inform nutrition policies in Iran. However, given its moderate diagnostic performance and the nature of available evidence, the WDI should not be used as a standalone clinical diagnostic tool.