BACKGROUND We assessed nationwide trends in extreme obesity prevalence and its impact on metabolic syndrome in Korean youth. METHODS Data from 19,593 youth (10,271 boys) aged 2-19 were obtained from the Korean National Health and Nutrition Examination Surveys (2001-2014). The prevalence of extreme obesity and the odds ratio of metabolic syndrome were calculated. Questionnaires on body weight perceptions and weight control behaviors were administered. RESULTS The prevalence of extreme obesity increased significantly among boys (p<0.05). The odds ratios of extreme obesity on metabolic syndrome compared with obesity were 3.781 and 1.979 in boys and girls, respectively. Obese teenage girls with appropriate weight perception attempted to lose weight more frequently than obese teenage boys did. CONCLUSIONS Although the prevalence of childhood obesity in Korea was similar since early 2000s, extreme obesity prevalence has increased, especially among teenage boys. Considering greater morbidity risks, preventing progression to extreme obesity is crucial.