Given the prevalence and undesirable consequences of smartphone dependency among adolescents, it is necessary to explore the influencing factors of adolescent smartphone dependency. The aim of this study was to examine the intergenerational transmission of mother–adolescent smartphone dependency and the mediating role of negative parenting, moderated by adolescent gender. Data for 2541 middle school students (mean aged = 13 years)–mother dyads were obtained from the first wave of the Korean Children and Youth Panel Survey 2018 (KCYPS 2018). The moderated mediation model using Hayes PROCESS macro (Model 14) was employed to test the study hypothesis. The moderated mediation model revealed that maternal smartphone dependency was associated with adolescent smartphone dependency. Perceived negative parenting mediated this link and adolescent gender moderated the relationship between negative parenting and adolescent smartphone dependency, especially for adolescent girls. Our findings showed that both maternal smartphone dependency and negative parenting were determinants of adolescent smartphone dependency, suggesting that both factors were important for understanding these issues. Moreover, the mediating role of negative parenting (between maternal and adolescent smartphone dependency) implies that parental education programs designed to improve negative parenting may reduce adolescent smartphone dependency, especially for adolescent girls.