BACKGROUND AND OBJECTIVES: Physical activity is associated with social network site addiction in adolescents, yet the mechanisms remain unclear. This study examines whether anxiety and ego-depletion mediate this relationship. METHODS: A survey was conducted among 1,174 Chinese adolescents (614 boys, 560 girls; mean age = 12.59, SD = 1.13). Physical activity was assessed with a single item on moderate-to-vigorous exercise in the past 7 days. Social network site addiction, anxiety, and ego-depletion were measured using validated self-report questionnaires. Descriptive statistics, correlation analyses, and a chained mediation model were employed. RESULTS: Physical activity was negatively correlated with social network site addiction (r = -0.165, p < 0.001), anxiety (r = -0.121, p < 0.001), and ego-depletion (r = -0.119, p < 0.001). Anxiety was positively correlated with ego-depletion (r = 0.574, p < 0.001) and social network site addiction (r = 0.388, p < 0.001). Ego-depletion was positively associated with social network site addiction (r = 0.456, p < 0.001). Anxiety and ego-depletion sequentially mediated the relationship between physical activity and social network site addiction. CONCLUSION: This study clarifies the psychological mechanisms linking physical activity and social network site addiction in adolescents, identifying anxiety and ego-depletion as key mediators. The findings emphasize the need to target these factors for more effective interventions.