The hormonal contraceptive medroxyprogesterone acetate (MPA) is associated with increased risk of human immunodeficiency virus (HIV), via incompletely understood mechanisms. Increased diversity in the vaginal microbiota modulates genital inflammation and is associated with increased HIV-1 acquisition. However, the effect of MPA on diversity of the vaginal microbiota is relatively unknown. In a cohort of female Kenyan sex workers, negative for sexually transmitted infections (STIs), with Nugent scores <7 (N=58 of 370 screened), MPA correlated with significantly increased diversity of the vaginal microbiota as assessed by 16S rRNA gene sequencing. MPA was also significantly associated with decreased levels of estrogen in the plasma, and low vaginal glycogen and α-amylase, factors implicated in vaginal colonization by lactobacilli, bacteria that are believed to protect against STIs. In a humanized mouse model, MPA treatment was associated with low serum estrogen, low glycogen and enhanced HIV-1 susceptibility. The mechanism by which the MPA-mediated changes in the vaginal microbiota may contribute to HIV-1 susceptibility in humans appears to be independent of inflammatory cytokines and/or activated T cells. Altogether, these results suggest MPA-induced hypo-estrogenism may alter key metabolic components that are necessary for vaginal colonization by certain bacterial species including lactobacilli, and allow for greater bacterial diversity in the vaginal microbiota..