Background: Problematic smartphone usage has become a worldwide problem due to its high prevalence, rapid growth, and serious adverse consequences. Objective: The purpose of this study was to explore the mediating role of depressive symptoms in the relationship between stressful life events and problematic smartphone usage and the protective role of peer support between stressful life events and depressive symptoms in boarding-school adolescents. Method: 124 rural boarding-school adolescents (61 boys, 49%) completed questionnaires about stressful life events (wave 1), peer support (wave 1), depressive symptoms (wave 2), and problematic smartphone usage (wave 3) between 2016 and 2018. Results: A structural equation modeling and multi-group analysis showed that (1) depressive symptoms mediated the effect of stressful life events on later problematic smartphone usage; (2) peer support had a negative effect on depressive symptoms, but it did not moderate the relationship between stressful life events and depressive symptoms; and (3) no gender difference were found on these associations. Conclusions: Depressive symptoms fully mediated the association between stressful life events and problematic smartphone usage among boarding-school adolescents. Peer support can alleviate depressive symptoms, thereby reducing the risk of problematic smartphone usage. [ABSTRACT FROM AUTHOR]