NR5A2 is an orphan nuclear receptor involved in cholesterol metabolism and embryogenesis. The high level of expression of NR5A2 in the ovary and its involvement in the regulation of steroidogenic gene expression also suggest a role for this transcription factor in female reproductive function. In vivo evidence for a role for NR5A2 in fertility, however, is still lacking. In order to address this possibility, we used Nr5a2 ^/− mice to demonstrate that heterozygosity for a null mutation of Nr5a2 leads to a decreased fertility in females. Our results indicate that although Nr5a2 ^/− mice display normal follicular development, ovulation, and estrogen production, they exhibit altered luteal function. More specifically, we show that the reduced reproductive ability of Nr5a2 ^/− females arises from a reduction in circulating progesterone concentrations and can be rescued by exogenous progesterone supplementation. This study therefore provides the first in vivo evidence for a role of NR5A2 in reproductive function and steroidogenesis.