This study aimed to describe physical activity (PA) and sitting time, and to examine associations between sociodemographic factors, weight status, PA, and sitting time among South Korean adolescents (12-18 years). Findings are based on self-report data from 638 participants in the 2013 Korea National Health and Nutrition Examination Survey. Only 4.9% of adolescents accumulated 60 minutes of moderate-to-vigorous PA daily. Adolescents spent 532.4 ± 9.3 mins/d sitting. After controlling for age and sex, individuals in higher income groups compared with the lowest income group, living in nonmetro Seoul compared with metro Seoul, and who were overweight compared with nonoverweight were more likely to meet PA guidelines. Participants in the highest income group compared with lowest income group, and residing in nonmetro Seoul compared with metro Seoul were more likely to be in the high sitting time group (>720 min/d) (P < .05). Increasing PA and reducing sitting should be a public health priority in South Korea.