Studies suggest that long-chain ?-3 polyunsaturated fatty acid (LC?3PUFA) intake and its primary food source-fish-may have beneficial effects on the individual components of metabolic syndrome (MetS). We examined the longitudinal association between fish or LC?3PUFA intake and MetS incidence. We prospectively followed 4356 American young adults, free from MetS and diabetes at baseline, for incident MetS and its components in relation to fish and LC?3PUFA intake. MetS was defined by the National Cholesterol Education Program/Adult Treatment Panel III criteria. Cox proportional hazards model was used for analyses, controlling for socio-demographic, behavioral, and dietary factors. During the 25-year follow-up, a total of 1069 incident cases of MetS were identified. LC?3PUFA intake was inversely associated with the incidence of MetS in a dose-response manner. The multivariable adjusted hazards ratio (HR) [95 % confidence interval (CI)] of incident MetS was 0.54 (95 % CI 0.44, 0.67; P for linear trend < 0.01) as compared the highest to the lowest quintile of LC?3PUFA intake. A threshold inverse association was found between non-fried fish consumption and the incidence of MetS. The multivariable adjusted HRs (95 % CIs) from the lowest to the highest quintile were 1.00, 0.70 (0.51, 0.95), 0.68 (0.52, 0.91), 0.67 (0.53, 0.86), and 0.71 (0.56, 0.89) (P for linear trend = 0.49). The observed inverse associations were independent of the status of baseline individual components of MetS. Our findings suggest that intakes of LC?3PUFAs and non-fried fish in young adulthood are inversely associated with the incidence of MetS later in life.