The study's aim was to investigate the trajectory of smartphone dependency and to find the associated factors predicting smartphone dependency. Methods: Data were obtained from the fourth (2013) to the seventh (2016) Korean Children and Youth Panel Survey studies (N = 1,248). The trajectory of smartphone dependency was analysed using the Latent Growth Curve Model with SPSS 21.0 and AMOS 20.0. Results and Conclusions: Over the four-year period, smartphone dependency showed a linear increase along with significant individual differences in intercept and slope. For girls, the trajectory of smartphone dependency, higher depression, increased computer and game time, parental abuse, affectionate and inconsistent parenting, and lower academic attitude had significant effects and showed a higher degree of intercept. Escalating trajectory patterns were associated with identification of predictors and early development of smartphone dependency. The factors identified may be useful in the development of intervention for smartphone dependency. The theoretical and practical implications of the findings are discussed.