This study examined the relationships among parental smartphone addiction, adolescents’ media device addiction, depression, and physical activity, focusing on differences between fathers and mothers. Using data from the 15th wave of the Korean Children and Youth Panel Study, responses from 1,142 adolescents (589 boys, 553 girls) and their parents were analyzed. Moderated mediation analyses were conducted to test whether adolescents’ media device addiction mediated the link between parental smartphone addiction and adolescent depression, and whether adolescents’ physical activity moderated this indirect effect. Results indicated that fathers’ smartphone addiction did not directly predict adolescents’ media device addiction; however, its effect was significant when adolescents’ physical activity was at medium or high levels, showing a significant moderated mediation effect. In contrast, mothers’ smartphone addiction had a direct positive effect on adolescents’ media device addiction, which in turn predicted higher depression, but the mediation pathway was not moderated by physical activity. For both parents, the effect of smartphone addiction on adolescent depression was entirely indirect, mediated by adolescents’ media device addiction. These findings highlight the differential roles of fathers’ and mothers’ smartphone use in adolescents’ media habits and mental health, suggesting that intervention programs should consider parental gender and adolescents’ activity contexts.