Vascular function is reportedly influenced by vitamin D and is worse in African Americans (AAs) relative to European Americans (EAs). Reasons remain unclear especially if ethnic differences in 25-hydroxyvitamin D [25(OH)D] levels mediate differences in vascular function. This prospective study investigated the relationships of serum 25(OH)D with indices of vascular function among 45 healthy, 18–50 yo AA and EA adults. The main outcomes were augmentation index (AIx75), central aortic pressure, pulse wave velocity (PWV), flow-mediated dilation (FMD), and seated and supine blood pressures. Results showed that 25(OH)D was inversely associated with AIx75, supine systolic blood pressure (SBP), central aortic SBP and central aortic diastolic blood pressure (DBP), independent of age, sex, and percent body fat (P<0.05 for all). 25(OH)D was associated with AIx75, PWV and FMD (P=0.05 to 0.08) among AAs, but not EAs (P=0.44 to 0.96). AAs had greater AIx75 (P=0.04) and PWV (P=0.07) and lower FMD (P=0.02). The relationship between age and PWV was significant in low 25(OH)D versus normal 25(OH)D status. In conclusion, 25(OH)D may have favorable effects on vascular function, particularly AAs, and lower 25(OH)D among AAs may contribute to their greater arterial stiffness and endothelial dysfunction. Furthermore, adequate vitamin D levels may attenuate age-related increases in arterial stiffness.