<p>This study examined the mechanisms underlying smartphone addiction (SA) among adolescents, focusing on parental psychological aggression (PPA) as a critical risk factor. Drawing on the Interaction of Person-Affect-Cognition-Execution (I-PACE) model, the study investigated the mediating role of loneliness and the moderating role of physical activity (A) in this pathway. A sample of 911 adolescents (mean age = 16.43 years, SD = 1.32; 53.8% female) was recruited from 15 schools across South Korea between September and December 2023. The direct effect of parental psychological aggression on smartphone addiction became non-significant after including loneliness as a mediator (B = 0.150, 95% CI [0.067, 0.246]), suggesting that loneliness mediates this association. Furthermore, physical activity moderated the first stage of this mediation, such that the indirect effect was significant at low (B = 0.205, 95% CI [0.106, 0.340]) and average (B = 0.145, 95% CI [0.065, 0.229]) levels of physical activity, but non-significant at high levels (B = 0.085, 95% CI [-0.073, 0.179]). These findings indicate that loneliness is a key mechanism linking parental psychological aggression to smartphone addiction, and that physical activity may serve as a protective factor attenuating this pathway. The results underscore the value of school-based physical activity programmes as accessible interventions for adolescents at risk of smartphone addiction.</p>