The objective of the study was to evaluate the association between gestational weight gain, per the 2009 Institute of Medicine (IOM) recommendations, and offspring overweight/obesity at 2-5 years of age. This was a prospective cohort study of 4145 women who completed a health survey (2007-2009) and subsequently delivered a singleton at Kaiser Permanente Northern California (2007-2010). Childhood overweight/obesity was defined as a body mass index (BMI) z-score of the 85th percentile or greater of the Centers for Disease Control and Prevention child growth standards. Gestational weight gain was categorized according to the 2009 IOM recommendations. Logistic regression was used; meeting the IOM recommendations was the referent. Exceeding the IOM recommendations was associated with a 46% increase in odds of having an overweight/obese child (odds ratio [OR], 1.46; 95% confidence interval [CI], 1.17-1.83), after adjusting for maternal prepregnancy BMI, race/ethnicity, age at delivery, education, child age, birthweight, gestational age at delivery, gestational diabetes, parity, infant sex, total metabolic equivalents, and dietary pattern. The OR (95% CI) for childhood overweight/obesity among women gaining below the IOM recommendations was 1.23 (0.88-1.71). The associations between gaining outside the IOM recommendations and childhood obesity were stronger among women with a normal prepregnancy BMI (OR, 1.63; 95% CI, 1.03-2.57) (below); OR, 1.79; 95% CI, 1.32-2.43) (exceeded). Gestational weight gain outside the IOM recommendations is associated with increased odds of childhood overweight/obesity, independent of several potential confounders and mediators. Gestational weight gain had a greater impact on childhood overweight/obesity among normal-weight women, suggesting that the effect may be independent of genetic predictors of obesity.