The global rise in smartphone addiction among children is linked to increasing mental, physical, familial, and financial problems. This study aimed to longitudinally explore the impact of sibling smartphone addiction on child smartphone addiction. While the influences of parental smartphone addiction and peer groups on smartphone addiction have been studied, more research is needed to examine the longitudinal effects of sibling smartphone addiction, a significant family member during childhood, on child smartphone addiction.This study conducted a longitudinal analysis using four years of panel data (2019-2022) from the Korean Children and Youth Panel Survey (KCYPS). The final analysis included a total of 1,978 participants, comprising fourth-grade elementary school children (n=989) and their siblings (n=989). The changes in smartphone addiction over the four years were analyzed using latent growth analysis.First, while the rate of change in child smartphone addiction was higher, both sibling and child smartphone addiction increased over time. Lower initial smartphone addiction levels were linked to a greater increase in smartphone usage over time. These results indicate the need for interventions targeting groups with lower initial levels of smartphone addiction. Second, higher initial levels of sibling smartphone addiction were associated with greater child smartphone addiction, and the change rate of sibling smartphone addiction influenced that of child smartphone addiction. As sibling smartphone addiction sharply increased over time, child smartphone addiction similarly escalated dramatically. On the other hand, it was found that as sibling smartphone addiction increases gradually, child smartphone addiction also increases gradually.This study identified the longitudinal impact of sibling smartphone addiction on the onset and progression of child smartphone addiction. The findings suggest that when preventing and intervening in the risk of child smartphone addiction, it is effective to consider the issue of sibling smartphone addiction in a long-term context.