Background: Physical activity is known to prevent several diseases and positively affect mental health. Previous studies have shown that smartphone addiction negatively affects the physical activity of children and adolescents. This study aimed to investigate the relationship between problematic smartphone use and physical activity among adolescents and the related factors using path analysis. Methods: Using data from the 16th Youth Risk Behavior Web-based Survey from 2020, scores on the Smartphone Addiction Scale—Short Version for Adolescents, physical activity, sex, socioeconomic status (SES), academic performance, depression, smoking, drinking, and sitting time were assessed. Complex sampling and path analyses were performed. Results: Of the total 54,948 students, 25.5% were smartphone risk users, including potential and high-risk users. The direct path coefficients of each factor indicated that female sex (-0.14 for male), low SES (-0.062), high academic performance (0.056), low sitting time for studying purposes (-0.033), high sitting time for non-studying purposes (0.071), and depressive mood (0.130) were related to problematic smartphone use (all P<0.001). Each factor affected problematic smartphone use, and subsequently had a negative effect on the amount of physical activity, with a direct path coefficient of -0.115 (P<0.001). Conclusion In this study, we confirmed that problematic smartphone use among adolescents was negatively associated with performing an adequate amount of physical activity and that various factors, such as sex, SES, academic performance, and sitting time, directly and indirectly affected this relationship.