<p>The advent of the digital age has given rise to smartphone addiction, which has a profound impact on adolescents’ psychological well-being and behavior. Based on resource depletion theory, the experiential avoidance model, and the cognitive-emotional model, the present study examines the longitudinal association between smartphone addiction and non-suicidal self-injury (NSSI) in adolescents, as well as the mediating role of negative emotions and the moderating effect of distress tolerance. Participants were 1,531 Chinese adolescents (50.6% male; <i>M</i>_age at Wave 1 = 13.25 years) recruited for a two-wave longitudinal study conducted over a six-month period. The results indicated that, after controlling for gender, age, and baseline levels of negative emotions and NSSI, negative emotions mediated the association between smartphone addiction and NSSI six months later. Moreover, this indirect relationship was moderated by distress tolerance. Specifically, the mediation effect weakened as adolescents’ distress tolerance increased. These findings enhance our understanding of the psychological mechanisms linking smartphone addiction to NSSI and provide valuable guidance for developing emotion-focused prevention and intervention strategies for adolescent NSSI.</p>