Background <p>Smartphone addiction represents a significant and growing public health challenge among adolescents. This longitudinal study examines the mediating role of social avoidance in the link between relative deprivation and smartphone addiction among Chinese junior high school students.</p> Methods <p>Data were collected from 639 participants over three waves at four-month intervals. By using a random intercept cross-lagged panel model (RI-CLPM), we were able to separate between-person and within-person effects.</p> Results <p>We found significant bidirectional predictive relationships between relative deprivation and smartphone addiction at the within-person level. Additionally, social avoidance served as a mediator in the longitudinal pathway from relative deprivation to smartphone addiction. Relative deprivation at T1 predicted increased social avoidance at T2, which then predicted higher levels of smartphone addiction at T3.</p> Conclusions <p>These results emphasize social avoidance as a key behavioral mechanism linking perceived inequality (i.e., relative deprivation) to problematic smartphone use. The study provides valuable insights for targeted interventions aimed at reducing smartphone addiction by addressing social avoidance behaviors in adolescents experiencing relative deprivation.</p>