Background: The protective role of (poly)phenols against metabolic disorders has been extensively studied in adults but not in adolescents. Objective: To assess associations of dietary (poly)phenols and their subclasses with cardiometabolic health parameters in adolescents. Methods: A cross-sectional study was conducted in 944 individuals aged 11 to 14 years enrolled in the SI! Program for Secondary Schools trial (NCT03504059). (Poly)phenol intake was assessed using semi-quantitative food frequency questionnaires and the Phenol-Explorer database. The measured cardiometabolic parameters were waist circumference (WC) age-sex z-score, blood pressure (BP) age-sex z-score, blood glucose (BG), triglycerides (TG), and high-density lipoprotein cholesterol (HDL-c). Multilevel mixed-effect linear regression models were applied to examine the association between (poly)phenol quintiles and cardiometabolic health parameters. Results: Compared to the lowest quintile, adolescents in the highest quintile of total (poly)phenol intake had lower WC z-scores (β=-0.09, 95%CI=-0.09;-0.08, P-for trend=0.002), mean arterial pressure z-scores (β =-0.11, 95%CI=-0.12;-0.09, P-for trend=0.206), and HDL-c (β=-1.52, 95%CI=-2.90;-0.14, P-for trend=0.108), after multivariable adjustment. The WC z-scores (β=-0.11, 95%CI=-0.19;-0.03, P-for trend=0.013) and HDL-c (β=-2.16, 95%CI=-3.78;-0.53, P-for trend=0.046) were lower in the highest quintile of flavonoid intake compared to the lowest quintile. The highest quintile of phenolic acid intake was associated with a lower WC z-score (β=-0.14,