This study aimed to explore the alleviating effects of fisetin, a polyphenolic flavonoid, on ovarian dysfunction in a D-galactose (D-gal)-induced aging mouse model, as well as the underlying mechanisms, using both in vivo and in vitro experiments. Mice were subcutaneously injected with D-gal (100 mg/kg/day) for 60 days to establish the ovarian aging model; during the final 30 days, fisetin (10, 20, 30 mg/kg/day) was given orally. In addition, a senescent model of granulosa cell (GC) was established using D-gal and treated with fisetin. Fisetin supplementation improved ovarian endocrine function and reproductive capacity in aging mice, as reflected by regularized estrous cycles, elevated estradiol levels, and increased embryo numbers.