Objectives: This study aimed to identify the influences of physical activity environments on obesity among adolescents. Methods: Data from 2017 Korea Youth Risk Behavior Web-based Survey were used for the analysis. Multivariate logistic regression analysis was conducted to analyze the effects of the physical activity during the classes at school on obesity. Stratified analysis was also conducted to compare the effects of physical activities at school according to the access to public parks and exercise facilities in the community. Results: Of the 60,392 respondents, 8,374 (13.9%) were obese. After controlling for their sociodemographic factors and health behaviors, the adolescents who experienced regular physical activities at school more than 3 times per week were less likely to be obese than their counterparts. According to the stratified analysis, the effects of school physical activity were still more significant in the community with lower access to public parks or exercise facilities. Conclusions: To prevent and manage obesity among adolescents, it is necessary to improve school environments facilitating physical activity and fostering the regular education curriculum with more frequency, intensity, and duration of physical activity.