Purpose: This study investigated the influence of sedentary behavior and smartphone use on the risk of generalized anxiety disorder among adolescents. Methods: This study was a cross-sectional, secondary analysis using the 2021 Korea Youth Risk Behavior Survey data. Data from 54,848 adolescents were analyzed using descriptive statistics, the Rao-Scott x2 test, and hierarchical logistic regression. Results: After controlling for socio-demographic, health-related behavior, and mental health factors as compounding variables, adolescents who sat for more than seven hours, when studying, on weekends had a significantly higher risk of generalized anxiety disorder odds ratios than those who sat for less than three hours. Adolescents who used smartphones for more than eight hours on weekdays and weekends had a significantly higher risk of generalized anxiety disorder odds ratios than those who used smartphones for less than four hours. Conclusion: It is necessary to provide an expanded health education program that reduces sedentary behavior and smartphone use to prevent the risk of generalized anxiety disorder at home and school.