Abstract Objective To assess whether maternal ultra-processed food intake during peripregnancy and during the child rearing period is associated with offspring risk of overweight or obesity during childhood and adolescence. Design Population based prospective cohort study. Setting The Nurses’ Health Study II (NHSII) and the Growing Up Today Study (GUTS I and II) in the United States. Participants 19 958 mother-child (45% boys, aged 7-17 years at study enrollment) pairs with a median follow-up of 4 years (interquartile range 2-5 years) until age 18 or the onset of overweight or obesity, including a subsample of 2925 mother-child pairs with information on peripregnancy diet. Main outcome measures Multivariable adjusted, log binomial models with generalized estimating equations and an exchangeable correlation structure were used to account for correlations between siblings and to estimate the relative risk of offspring overweight or obesity defined by the International Obesity Task Force. Results 2471 (12.4%) offspring developed overweight or obesity in the full analytic cohort. After adjusting for established maternal risk factors and offspring’s ultra-processed food intake, physical activity, and sedentary time, maternal consumption of ultra-processed foods during the child rearing period was associated with overweight or obesity in offspring, with a 26% higher risk in the group with the highest maternal ultra-processed food consumption (group 5) versus the lowest consumption group (group 1; relative risk 1.26, 95% confidence interval 1.08 to 1.47, P for trend<0.001). In the subsample with information on peripregnancy diet, while rates were higher, peripregnancy ultra-processed food intake was not significantly associated with an increased risk of offspring overweight or obesity (n=845 (28.9%); group 5 v group 1: relative risk 1.17, 95% confidence interval 0.89 to 1.53, P for trend=0.07). These associations were not modified by age, sex, birth weight, and gestational age of offspring or maternal body weight. Conclusions Maternal consumption of ultra-processed food during the child rearing period was associated with an increased risk of overweight or obesity in offspring, independent of maternal and offspring lifestyle risk factors. Further study is needed to confirm these findings and to understand the underlying biological mechanisms and environmental determinants. These data support the importance of refining dietary recommendations and the development of programs to improve nutrition for women of reproductive age to promote offspring health.