The purpose of this study was to identify factors associated with smartphone overdependency in preschool children based on an analysis of mother, child, and their mutual dyadic relationships. Methods: Data were collected from 171 mothers of preschool children enrolled at daycare centers and kindergartens from January to March 2018. The data were analyzed by descriptive statistics, the t-test, analysis of variance with Scheffé post hoc test, Pearson correlation coefficient, and hierarchical regression analysis using SPSS version 24. Results: The self regulation of preschool children (β=-.358, p<.001) was the factor most closely associated with smartphone overdependency. The second most closely associated factor was children's use of a smartphone 0.5-1 hour daily (β=-.249, p=.005). Additional associated factors were mothers' use of a smartphone for 3-4 hours daily (β=.217, p=.002), children's use of a smartphone for less than 0.5 hour daily (β=-.212, p=.006), and children's use of a smartphone for 1-2 days per week (β=-.205, p=.026). Conclusion: Based on the results of this study, it is necessary to try to develop guidelines and programs to prevent smart phone overdependency in preschool children.