To investigate the association between maternal obesity as measured by prepregnancy body mass index (BMI) and group B streptococcus (GBS) colonization. We conducted a secondary analysis from the Consortium on Safe Labor Study (CSL) in the United States cohort study (2002-2008). Pregnant women with deliveries at ≥37 weeks of gestation who attempted labor were included (115,070 assessed deliveries). The association between maternal prepregnancy BMI, categorized as normal weight or below (<25 kg/m), overweight (25 to <30 kg/m), class I obesity (30 to <35 kg/m), class II obesity (35 to <40 kg/m), and class III obesity (≥40 kg/m), and GBS colonization was modeled using logistic regression with generalized estimating equations. Models adjusted for maternal age, parity, race, pregestational diabetes, insurance status, study site/region, and year of delivery. The overall prevalence of GBS colonization was 20.5% (23,625/115,070), which increased with rising maternal BMI, normal weight 19.3% (13,543/70,098), overweight 20.8% (5,353/25,733), class I obesity 23.0% (2,596/11,275), class II obesity 26.1% (1,270/4,850), and class III obesity 27.7% (863/3,114). In multivariable analysis, increasing maternal obesity severity was associated with higher odds of GBS colonization, namely overweight (adjusted odds ratio [AOR]: 1.09, 95% confidence interval [CI]: 1.05-1.13), class I obesity (AOR: 1.20, 95% CI: 1.15-1.26), class II obesity (AOR: 1.42, 95% CI: 1.33-1.51), and class III obesity (AOR: 1.50; 95% CI: 1.38-1.62) compared with normal weight. In secondary analyses, these associations persisted when stratified by maternal race. In a national U.S. sample, increasing maternal obesity severity as assessed by prepregnancy BMI was associated with a higher likelihood of maternal GBS colonization during pregnancy.