Key PointsQuestionHow was the COVID-19 pandemic associated with early childhood development?FindingsThis cohort study in 447 children aged 1 to 3 years and 440 children aged 3 to 5 years in Japan found that children exposed to the pandemic were 4.39 months behind in development at age 5 years compared with those not exposed to the pandemic. Variations in development were greater during the pandemic vs the prepandemic period regardless of age.MeaningThese findings suggest the importance of identifying children with delayed development associated with the pandemic and providing them with support for learning, socialization, physical and mental health, and family support.