AIM: This study investigated the association between outdoor play habits during preschool and school-age obesity. METHODS: We conducted a longitudinal cohort study of all children born in Japan during 2 weeks in January and July 2001. We defined outdoor play habits at age 2.5 years (third survey) as exposure, while parent-reported height and weight at age 7 years (seventh survey) were defined as overweight and obesity status using the WHO reference. Logistic regression models were used to estimate odds ratios (ORs) for associations between preschool outdoor play habits and school-age obesity, adjusting for parental and child factors. RESULTS: Of 53 575 children born, 42 812 had data on outdoor play habits at age 2.5 years, with 91% (38 970) having such habits. At age 7 years, 31 743/42 812 (74%) children had height and weight data, with 3249/31 743 (10%) classified as overweight or obesity (BMI SD score ≥1.0). Outdoor play habits were negatively associated with obesity (adjusted OR 0.85, 95% confidence interval (CI): 0.74-0.97). CONCLUSION: Outdoor play habits in early preschool years are associated with a reduced risk of school-age obesity. Parents and caregivers may consider encouraging their children to outdoor play habits at an early age to help prevent obesity later in life.