Introduction: This study investigated associations between maternal prepregnancy BMI and child behaviors at ages 9-11 years and examine interaction by race and gender. Methods: The National Longitudinal Survey of Youth and the Children and Young Adults surveys are U.S.-based, ongoing longitudinal studies, initiated in 1979 and 1986, respectively. Mothers (n=2,952) reported pregnancy and child (n=5,660) developmental information at multiple time points. Child total, internalizing, and externalizing problems at ages 9-11 years were assessed using the Behavior Problems Index (BPI), collected biennially until 2012. Associations between prepregnancy BMI and child BPI outcomes were examined, as well as two- and three-way interactions by race and gender. Analyses were conducted in 2017. Results: Boys whose mothers had higher prepregnancy weights exhibited higher total BPI and externalizing scores at ages 9-11 years versus those with normal-weight mothers. Boys with severely obese mothers had higher total BPI (mean difference=7.99, 95% CI=3.53, 12.46) and externalizing (mean difference=5.77, 95% CI=1.50, 10.04) scores. Prepregnancy underweight was associated with boys’ higher total BPI (mean difference=2.34, 95% CI=0.02, 4.66) and externalizing (mean difference=3.30, 95% CI=0.69, 5.91); these associations were not significant in sensitivity analyses. No associations emerged for girls or internalizing problems. Two-way interactions by race and three-way interactions by race and gender were not significant. Conclusions: Maternal prepregnancy weight was associated with BPI level among boys. Boys with severely obese mothers exhibited markedly higher behavioral problems at ages 9-11 years versus those with normal-weight mothers, regardless of race. Maintaining healthy prepregnancy weight may be important for preventing boys’ deleterious behavior outcomes in middle childhood.