Although there is some evidence regarding an association between maternal bonding disorder and child development, studies have mainly focused on development during the period of infancy. We aimed to examine the associations between maternal postnatal bonding disorder and developmental delays in children beyond 2 years of age. We analyzed data from 8380 mother-child pairs who participated in the Tohoku Medical Megabank Project Birth and Three-Generation Cohort Study. Maternal bonding disorder was defined as Mother-to-Infant Bonding Scale score of ≥5 at 1 month after delivery. The Ages & Stages Questionnaires, Third Edition, which consists of five developmental areas, was used to assess developmental delays in children at 2 and 3.5 years of age. Multiple logistic regression analyses were conducted to examine the associations between postnatal bonding disorder and developmental delays after adjustment for age, education, income, parity, feelings toward pregnancy, postnatal depressive symptoms, child's sex, preterm birth, and birth defects. Bonding disorder was associated with developmental delays in children at 2 and 3.5 years of age: the odds ratios (95% confidence intervals) were 1.55 (1.32-1.83) and 1.60 (1.34-1.90), respectively. Bonding disorder was associated with delay in communication only at 3.5 years of age. Bonding disorder was associated with delay in gross motor, fine motor, and problem solving, but not delay in the personal-social domain, at 2 and 3.5 years of age. In conclusion, maternal bonding disorder 1 month after delivery was associated with an increased risk of developmental delays in children beyond 2 years of age.