Women are at significant risk of human immunodeficiency virus (HIV) and sexually transmitted infection (STI) acquisition with the genital mucosa serving as the main portal of infection. Exogenously supplied lactobacillus used as a probiotic may prove a cost-effective, female-initiated method to prevent HIV and STI infection in women. A probiotic may act indirectly through treating and preventing recurrent bacterial vaginosis or directly by secreting endogenous (e.g., hydrogen peroxide) and exogenous substances that block HIV and STI transmission. This review summarizes the preclinical and clinical studies that have been conducted so far to test probiotic bacteria for these purposes. Although significant progress has been made in this field, more fundamental research is required to better understand vaginal ecology to maximize probiotic formulations. Once identified, a suitable product will require testing in a well-designed, randomized, placebo-controlled trial to measure its effectiveness in augmenting antibiotic treatment to prevent bacterial vaginosis. If results from such a trial demonstrate efficacy, future studies should be designed to determine whether a probiotic can significantly lower the risk for HIV and STIs in at-risk female populations.