This study aimed to identify the association between sedentary behavior and anxiety disorders in 53,510 Korean adolescents. It analyzed data from the 16th (2020) Korea Youth Risk Behavior Web-based Survey (KYRBS). The dependent variable was the generalized anxiety disorder-7 (GAD-7). The GAD-7 scores were divided into normal, mild, moderate, and severe levels. The independent variables were sedentary time for learning, other sedentary times, total sedentary time, and regular physical activity. The confounding variables were sex, grade, stress, depression, substance abuse, suicidal thoughts, violent victimization, drinking, smoking, sleep satisfaction, and smartphone addiction. A chi-squared test, one-way analysis of variance, and logistic regression analysis were used for analysis. After adjusting for all confounding variables, the risk of severe level in GAD-7 increased by 1.045 times each time the sedentary time for learning based on increased by one hour. In other sedentary time and total sedentary time, the risk of severe level in GAD-7 increased by 1.025 times and 1.045 times per hour, respectively. However, in regular physical activity, after adjusting for the confounding variables, there was no significant association with the GAD-7 levels. Therefore, to prevent generalized anxiety disorders in Korean adolescents, it is necessary to reduce the overall sedentary times including sedentary time for learning.