Our objectives were to examine the interaction between maternal pre-pregnancy body mass index (BMI) and gestational weight gain (GWG) and their association with birthweight, with a focus on racial differences. We used birth certificate data from live singleton births of South Carolina resident mothers, who self-reported their race as non-Hispanic white (NHW, n = 140, 128) or non-Hispanic black (NHB, n = 82,492) and who delivered at 34-44 weeks of gestation between 2004 and 2008 to conduct a cross-sectional study. Linear regression was used to examine the relationship between our exposures (i.e., race, BMI and GWG) and our outcome birthweight. Based on 2009 Institute of Medicine guidelines, the prevalence of adequate, inadequate and excessive GWG was 27.1, 24.2 and 48.7%, respectively, in NHW women and 24.2, 34.8 and 41.0%, respectively, in NHB women. Adjusting for infant sex, gestational age, maternal age, tobacco use, education, prenatal care, and Medicaid, the difference in birthweight between excessive and adequate GWG at a maternal BMI of 30 kg/m(2) was 118 g (95% CI: 109, 127) in NHW women and 101 g (95% CI: 91, 111) in NHB women. Moreover, excessive versus adequate GWG conveyed similar protection from having a small for gestational age infant in NHW [OR = 0.64 (95% CI 0.61, 0.67)] and NHB women [OR = 0.68 (95% CI: 0.65, 0.72)]. In conclusion, we report a strong association between excessive GWG and higher infant birthweight across maternal BMI classes in NHW and NHB women. Given the high prevalence of excessive GWG even a small increase in birthweight may have considerable implications at the population level.