Objective: The purpose of this study was to examine the effects of young children’s emotionality on their smartphone overuse and to verify the mediation effect of mothers’ play participation on the relationship between young children’s emotionality and their smartphone overuse. Methods: Research participants include 274 mothers and their children aged 2 to 4 years old. The mothers completed questionnaires. Data analysis was performed with Pearson’s correlation coefficient and mediation analysis using SPSS 23.0. Results: The results of this study are summarized as follows. First, The young children’s emotionality showed a positive association with their smartphone overuse. Second, Their emotionality was negatively associated with their mothers play participation. Finally, the mothers’ play participation was found to partially mediate the relationship between young children’s emotionality and smartphone overuse. Conclusion/Implications: The results of this study suggest that it is necessary to provide parent education for supporting mothers’ play participation to prevent children from developing smartphone overuse.