Objective: This study aimed to examine the mediating effect of mothers’ parenting stress on the relationship between their self-differentiation and the tendency of addiction to social media. Methods: The participants in this study were two hundred and eight mothers with children aged 36 months or younger. Data were analyzed statistically using frequency, Pearson’s correlation coefficients, and hierarchical multiple regression analysis. Results: The results of the present study are as follows. First, the mothers’ self-differentiation was negatively associated with their parenting stress and social media addiction tendency. Second, both the total scores of the mothers’ parenting stress and their distress were found to partially mediate the relationship between their self-differentiation and social media addiction tendency. However, the mothers’ daily stress did not mediate the relationship between the variables. Conclusion/Implications: The findings of the current study have implications for developing ways of intervening in mothers’ social media addiction tendency by reducing their parenting stress, particularly for mothers with low levels of self-differentiation.