OBJECTIVE: Studies of body size and blood pressure (BP) in African American women typically focus on obesity overall or collapse obesity classes II and III into a single subgroup, ignoring potential heterogeneity in associations across categories. Moreover, ambulatory BP outcomes are primarily analyzed as mean daytime and/or nighttime BP, without examination of circadian changes during the day-to-night transition or the full 24-h cycle. METHODS: Functional data analysis methods were used to examine whether obesity categories modified ambulatory monitoring-assessed BP circadian rhythm in a cohort of 407 African American women. RESULTS: Age-adjusted systolic BP (SBP) was 4 mm Hg (95% CI: 0.4-8.4) higher among women with class I or II obesity than those with normal weight or overweight from 12:30 p.m. through 8:00 a.m. Age-adjusted differences in SBP among women with class III obesity versus those with normal weight or overweight were 6 mm Hg (95% CI: 0.7-10.8) during daytime hours and increased to 11 mm Hg (95% CI: 5.8-16.0) overnight. Compared with all other BMI categories, SBP of women with class III obesity declined more slowly from day to night. CONCLUSIONS: Circadian BP among African American women was distinct among those with class III obesity compared with those with other body weight categories, suggesting that intervention efforts in African American women should target this group.