Background: In recent years, the penetration rate of smartphones among Korean teenagers has increased, making it critical to clarify the influence of these devices on adolescents' lives. Objective: This study investigated the effects of smartphone dependence on peer relationships and life satisfaction among Korean adolescents. Methods: Using the middle school panel of the Korean Children and Youth Panel Survey (KCYPS), longitudinal data of 2,250 participants (53.4% boys) at Grade 1 (M[subscript age] = 14.01, SD = 0.03) in 2018, Grade 2 in 2019, and Grade 3 in 2020 were analyzed to examine the causal relationships between smartphone dependence, peer relationships, and life satisfaction. Results: This study found that smartphone dependence had a longitudinal effect on negative peer relationships. Negative peer relationships had a longitudinal negative effect on early adolescents' life satisfaction. Finally, negative peer relationships in early adolescents longitudinally mediated the relationship between smartphone dependence and life satisfaction. Conclusions: This study showed that early adolescents' dependence on smartphones negatively affects their social relationships and, in turn, negative relationships with peers hindered life satisfaction, suggesting that adolescents' smartphone use should be guided and, when necessary, receive assistance to develop control of their smartphone use.