Context: Ferritin concentration is associated with metabolic syndrome, but the possibility of a nonlinear association has never been explored. Objective: This study aimed to examine the relationship between serum ferritin levels and the metabolic syndrome in Spanish adult males. Design: This was a cross-sectional analysis of baseline data from the Aragon Workers' Health Study. Setting: Healthy workers from a factory were studied during their annual checkup. Participants: Spanish male adults (n = 3386) between the ages of 19 and 65 years participated. We excluded participants with ferritin > 500 μ/L, ferritin < 12 μg/L, or C-reactive protein > 10 mg/L. Main Outcome Measure: Metabolic syndrome was defined according to the 2009 consensus definition from the Joint Interim Statement of several international societies. Results: Metabolic syndrome prevalence was 27.1%. We found a positive association between elevated iron stores, measured as serum ferritin concentration, and metabolic syndrome and its criteria. Participants within the highest serum ferritin quintile had a higher risk than those in the lowest quintile for central obesity (odds ratio [OR], 1.88; 95% confidence interval [CI], 1.46-2.42), hypertriglyceridemia (OR, 2.15; 95% CI, 1.69-2.74), and metabolic syndrome (OR, 1.92; 95% CI, 1.48-2.49). The association was nonlinear and occurred at serum ferritin concentrations > 100 μg/L (∼ 33th percentile). Ferritin was also associated with insulin resistance, measured by homeostatic model assessment-insulin resistance (HOMA-IR) (P trend < .001). Conclusions: Our findings suggest that serum ferritin is significantly associated with metabolic syndrome and its criteria (especially central obesity and hypertriglyceridemia), suggesting that ferritin could be an early marker of metabolic damage in the development of metabolic syndrome.