<p>Dispositional mindfulness has been identified as a protective factor against psychological distress among adolescents, yet the psychological processes underlying this relationship remain unclear. Drawing upon the ecological system model, this study examined the mediating roles of school connectedness and friendship quality in the longitudinal relationship between dispositional mindfulness and psychological distress. A three-wave longitudinal design was implemented with 1,140 adolescents recruited from three high schools in Gansu and Xinjiang provinces, China. The findings revealed that (a) T1 Dispositional Mindfulness was negatively associated with T3 psychological distress, (b) the mediating effect of T2 Friendship Quality in the relationship between T1 dispositional mindfulness and T3 psychological distress was not significant, (c) T2 School Connectedness mediated the association between T1 dispositional mindfulness and T3 psychological distress, (d) T2 Friendship Quality and T2 School Connectedness played a chain mediating effect between T1 dispositional mindfulness and T3 psychological distress. These results highlight the importance of adolescents’ social environments in explaining how dispositional mindfulness protects against psychological distress. By identifying school connectedness and friendship quality as key mechanisms, the study provides insights for designing targeted interventions that strengthen adolescents’ social support systems to reduce psychological distress.</p>