Parenting stress is affected by various factors, including maternal attachment; however, the number of studies focusing on Japanese samples is limited. As such, we explored the association between mothers’ attachment styles and parenting stress among Japanese mothers with 18-month old toddlers. This cross-sectional study was conducted in Sasebo City, Japan between 2018 and 2019. Anonymous self-reported questionnaires were distributed to 1399 mothers who attended an infant health check-up. We categorized maternal attachment style as secure, anxious/ambivalent, or avoidant, and conducted a multiple logistic regression analysis to evaluate the associations between each attachment style and parenting stress. Of the 1399 mothers, 529 responded to the survey (37.8%). About 40% reported experienced parenting stress. Further, approximately two-thirds showed a secure attachment style, 20% had an anxious/ambivalent style, and 15% had an avoidant style. In the multiple logistic regression analysis, the ambivalently attached mothers had a significantly higher level of parenting stress than those with secure attachment (odds ratio = 2.4, 95% confidence interval (1.5, 3.9)), but avoidantly attached mothers did not have a significantly higher level of parenting stress than those with secure attachment (odds ratio = 0.9, 95% confidence interval (0.5, 1.6)). The findings demonstrate that an anxious/ambivalent attachment style is associated with a higher level of parenting stress than a secure style. Thus, it is important for experts to understand the mother’s attachment style when offering childrearing support.