Obesity is considered detrimental to women's reproductive health. Although most of the attention has been focused on the effects of obesity on hypothalamic function, studies suggest a multifactorial impact. In fact, obesity is associated with reduced fecundity even in women with regular cycles, indicating that there may be local ovarian effects modulating fertility. Here we describe a novel mechanism for leptin actions directly in the ovary that may account for some of the negative effects of obesity on ovarian function. We find that normal cycling, obese, hyperleptinemic mice fed with a high-fat diet are subfertile and ovulate fewer oocytes compared with animals fed with a normal diet. Importantly, we show that leptin induces expression of the neuropeptide cocaine- and amphetamine-regulated transcript (CART) in the granulosa cells (GCs) of ovarian follicles both in vitro and in vivo. CART then negatively affects intracellular cAMP levels, MAPK signaling, and aromatase mRNA expression, which leads to lower estradiol synthesis in GCs and altered ovarian folliculogenesis. Finally, in human samples from patients undergoing in vitro fertilization, we show a significant positive correlation between patient body mass index, CART mRNA expression in GCs, and CART peptide levels in follicular fluid. These observations suggest that, under obese conditions, CART acts as a local mediator of leptin in the ovary to cause ovarian dysfunction and reduced fertility.