Excessive androgens serve as a primary driver for many phenotypic features of polyendocrine metabolic ovarian syndrome (PMOS). Although dietary fiber has been shown to alleviate PMOS, its effects on androgen concentrations and the underlying mechanisms remain unclear. In a clinical trial, we demonstrated that a dietary fiber combination consisting of inulin, resistant starch, and β-glucan reduces androgen concentrations in patients' plasma. Mechanistically, dietary fiber increased the abundance of Bacteroides xylanisolvens, which converts androstenedione into a nonandrogenic secondary hormone metabolite we named coproandrosterone, which is further metabolized by gut microbiota into dihydro-coproandrosterone. This metabolic pathway was validated in a mouse model. These findings highlight the potential of dietary fiber to alleviate PMOS by modulating androgen homeostasis through gut microbiota, offering insights for prebiotic and probiotic therapies.