This study was conducted to identify the trends in obesity prevalence among adolescents and changes in the risk factors related to obesity. The study analyzed secondary data obtained from Korean Youth Risk Behavior Web-based Surveys conducted from 2009 to 2019. The Korean Youth Risk Behavior Web-based Survey is an annual survey of a nationwide representative sample of enrolled students aged 13-18 years in middle and high schools in Korea. Linear and trend analyses showed that the prevalence of obesity increased by 0.47% on average annually; this increase was statistically significant. Healthy food intake decreased significantly, but the prevalence of unhealthy food intake and the prevalence of skipping breakfast increased significantly. Vigorous-intensity physical activity, physical activity for over 60 min a day, and muscle-strengthening exercise for more than 3 days a week increased significantly, but so did the adolescents' sedentary time. Therefore, health care providers and public policymakers need to actively manage adolescent obesity, which has been continuously increasing since 2009. In addition, long-term trends in obesity-related risk factors such as physical activity and dietary behaviors need to be considered in the development of obesity management strategies.
