This study investigated the developmental trajectory of inattention symptoms as a predictor of later depressive symptoms in adolescence, and examined potential mediating role of peer relationships in this process. Participants were adolescents who were part of the large longitudinal panel study on Korean Youths, Korean Children & Youth Panel Survey 2010 of the National Youth Policy Institute. Specifically, data were drawn from two cohorts of KCYPS that differed in participant age. We analyzed data collected from 2010 to 2016 when children (...) in panel 1 were 6–7 to 12–13 years old, and children in panel 2 were 9–10 to 15–16 years old. Results of latent growth modeling were as follows. In Panel 1, the inattention symptoms increased from 9–10 to 12–13 years. Otherwise, the inattention symptoms decreased from 11–12 to 15–16 years in panel 2. Additionally, in both panels, initial status and slope of inattention significantly predicted later levels of depressive symptoms, and peer relationships partly mediated the association between inattention trajectory and depressive symptoms. The findings are discussed with respect to considering the growth of inattention and the quality of peer relationships as promising targets for early identification and intervention of depression in adolescents. (shrink)