Background: Social media use among adolescents has rapidly expanded, raising concerns about problematic social media use (PSMU). Few studies have examined how relative deprivation (RD) and contextual inequality shape PSMU. We explain whether four key interpersonal relationships (relationships with teachers, classmates, and family, and bullying victimization) mediate the link between RD and PSMU and whether school-level inequality moderates these associations. Method: The data was from the 2017/2018 Health Behavior in School-aged Children (190,707 adolescents, 5,576 schools, 39 countries/regions). The independent variable was RD, calculated via the Yitzhaki index, and the dependent variable was PSMU. Four interpersonal relationships (teacher, classmate, and family relationships, and bullying victimization) were assessed as mediators. Multilevel mediation models were used to evaluate the mediating roles of these relationships. We then extracted each country/region's indirect effects for cross-national comparisons. The Johnson-Neyman method was used to assess the moderating effect of contextual inequality. Results: Although RD had no direct effect on PSMU, higher RD was associated with poorer teacher, classmate, and family relationships and heightened bullying victimization. These then predicted increased PSMU. Contextual inequality strengthened RD's adverse association with family relationships and magnified the associations of family relationships and bullying on PSMU. Cross-national analyses revealed stable mediating effects of teacher and classmate relationships. Conclusions: We highlight RD's associations with PSMU primarily through interpersonal pathways, with contextual inequality amplifying them. Targeted interventions that address both socioeconomic disparities and interpersonal dynamics can help mitigate PSMU. These outcomes underscore the need for policies that reduce economic gaps and foster supportive relationships.  Kulcsszavak: relative deprivation, problematic social media use, interpersonal relationships, contextual inequality, multilevel mediation, cross-national comparison