OBJECTIVES: To determine the association between mothers' persistent maternal depression and their toddlers' behavior. METHODS: Online surveys were conducted twice with mothers who gave birth to their first child between March and June 2020. The survey periods were November 2020 and May-June 2022. Measures included baseline characteristics and family environment factors, maternal postpartum depression (Edinburgh Postnatal Depression Scale [EPDS]), maternal lifestyle and mother-reported toddler behaviors, and Internet/media use. Statistical analysis was performed using the χ² test, trend test, and logistic regression. RESULTS: Of the 339 participants, 82 (24.1%) were in the "persistent maternal depression" group with high EPDS scores (≥ 9 points) at both time points, and 178 (52.5%) were in the "no maternal depression" group with low EPDS scores (<
9 points) at both time points. Persistent maternal depression was associated with sleep, eating behavior, physical activity, parenting emotions, and abusive behavior of mothers. Furthermore, persistent maternal depression may be related to undesirable toddler behaviors such as smartphone overuse and hyperactivity-like symptoms. CONCLUSIONS FOR PRACTICE: The persistence of postpartum depression is influenced by factors such as mothers' parenting emotions. Preventing and supporting maternal depression in mothers can foster favorable behaviors in toddlers. In Japan, enhanced individualized, ongoing support for postpartum mothers, tailored in duration and form, could promote both maternal well-being and positive parenting practices.