BACKGROUND: Chronic inflammation is associated with noncommunicable diseases, including obesity, metabolic syndrome (MetS), and CVDs. The Mediterranean diet has been shown to have strong anti-inflammatory effects, attributed in part to the polyphenol richness of many of its components. OBJECTIVES: This study aimed to assess the value of polyphenols as a urinary biomarker of an anti-inflammatory diet and their influence on MetS status. METHODS: A longitudinal analysis was performed in Spain considering 543 participants with high CVD risk in a PREDIMED study. Approximately 52% of the participants were women and 48% were men with a mean age of 67.5 (5.9) y. Total polyphenol excretion (TPE) in urine was determined at baseline and 5 y of intervention using a validated Folin-Ciocalteu spectrophotometric method, and the dietary inflammatory index (DII) was calculated from a validated 137-item food-frequency questionnaire. Three categories were built according to tertiles of change in the DII score. Multivariable linear regression analyses were performed to assess the association of changes in TPE with changes in the DII scores and with MetS status at 5 y. RESULTS: Tertiles 2 and 3 compared with tertile 1 presented a lower anti-inflammatory potential of the diet and were inversely associated with TPE in women [-0.30 mg gallic acid equivalent (GAE)/g creatinine; 95% CI: -0.46, -0.15; P value = 0.006 and -0.29 mg GAE/g creatinine; 95% CI: -0.43, -0.15; P value = 0.005], respectively. The mean changes in TPE were 7.9 (56.1) mg GAE/g creatinine in women and 7.7 (48.2) mg GAE/g creatinine in men. In addition, TPE was inversely associated with changes in MetS status [-0.06 (-0.09; -0.02), P value = 0.009] in both men and women. CONCLUSIONS: Urinary polyphenols may be a potential biomarker of anti-inflammatory diet consumption in women and are prospectively associated with improvement in MetS.