Objective: Adult overweight is associated with increased risk of diverticular disease (DD). We investigated associations between birthweight and childhood body mass index (BMI) and DD. Methods: Cohort study of 346,586 persons born during 1930–1996 with records in the Copenhagen School Health Records Register. Data included birthweight, and height and weight from ages 7 through 13. We used Cox proportional hazard regression to examine associations between birthweight and BMI z-scores and DD registered in the Danish National Patient Registry. Due to non-proportionality, we followed participants from age 18–49 and from age 50. Results: During follow-up, 5459 (3.2%) women and 4429 (2.5%) men had DD. For low and high BMI in childhood, we observed a higher risk of DD before age 50. Among women with z-scores <0 at age 13, the hazard ratio (HR) was 1.16 [95% confidence interval (CI): 0.98–1.39] per one-point lower z-score. For z-scores ≥0 at age 13, the HR was 1.30 (95% CI: 1.11–1.51) per one-point higher z-score. Among men with z-scores <0 at age 13, the HR was 1.02 (95% CI: 0.85–1.22). For z-scores ≥0 at age 13, the HR was 1.54 (95% CI: 1.34–1.78). Z-scores ≥0 were not associated with DD after age 50. Among women only, birthweight was inversely associated with DD before age 50 [HR = 0.90 (95% CI: 0.83–0.99) per 500 g higher birthweight]. Conclusion: BMI z-scores below and above zero in childhood were associated with higher risk of DD before age 50. In addition, we observed lower risk of DD among women, the higher their birthweight.