Concomitantly with the increasing prevalence of childhood obesity, the prevalence of metabolic syndrome (MetS) is rising among children and adolescents, leading to fears for future epidemics of type 2 diabetes mellitus and cardiovascular disease in the young. MetS includes central obesity, hypertension, low HDL cholesterol, high triglycerides, and elevated concentrations of fasting blood glucose. Nonalcoholic fatty liver disease (NAFLD), which is the most common cause of chronic liver disease both in adults and children, is currently considered as the hepatic component of MetS. Growing evidence suggests that n-3 long-chain polyunsaturated fatty acids (LC-PUFAs) may have a beneficial role on many factors associated with MetS, including circulating lipids and low-grade chronic inflammation. Recently, there has also been growing interest in n-3 LC-PUFAs supplementation as potential treatment for NAFLD, which is pathogenically linked to insulin resistance and involves inflammation in its advanced stages. Problems of adherence to lifestyle interventions, especially in children, and side effects with pharmacological agents make the dietary fish oil supplementation a simple and practical alternative therapy. Fish oil provides a convenient source of essential n-3 LCPUFA with few side effects, and may directly decrease circulating lipids, hepatic lipogenesis and steatosis, and chronic inflammation. In this review, we examine observational and interventional studies assessing in children and adolescents the potential impact of dietary n-3 PUFA supplementation on MetS components including NAFLD, and discuss the mechanisms underlying the actions of n-3 LC- PUFA on multiple risk factors associated with MetS.