Previous studies found conflicting results about the associations between the exposure to hyperglycemia in utero and the later risks of childhood overweight and obesity. The aim of the present study is to compare the children’s BMI growth between offspring exposed to maternal gestational diabetes mellitus (GDM) and those not exposed and assess the associations between maternal GDM and their offspring’s overweight and obesity risk. We performed a large observational study in 1156 women and their offspring (578 GDM and 578 non-GDM mother–child pairs, matched by their offspring’s gender and age). Maternal GDM was diagnosed according to the World Health Organization criteria. Childhood height, weight, waist circumference, body fat and skinfold were measured using standardized methods. After adjustment for maternal and children’s characteristics, children born to mothers with GDM during pregnancy had higher mean values of Z scores for BMI-for-age, Z scores for weight-for-age, waist circumferences, body fat, subscapular skinfold and suprailiac skinfold, in comparison with their counterparts born to mothers with normal glucose during pregnancy (all P values <0.05). Moreover, maternal GDM was associated with a higher risk of childhood overweight and obesity with multivariate-adjusted odds ratios of 1.42 (95% confidence interval (CI): 1.02–1.97) and 1.18 (95% CI: 1.11–1.24), respectively, compared with the children of mothers without GDM during pregnancy. This study demonstrates that maternal GDM is an independent risk factor of childhood overweight and obesity and is associated with higher BMI in the offspring.