We found a novel sexually dimorphic area (SDA) in the dorsal hypothalamus (DH) of mice. The SDA-DH was sandwiched between 2 known male-biased sexually dimorphic nuclei, the principal nucleus of the bed nucleus of the stria terminalis and the calbindin-sexually dimorphic nucleus, and exhibited a female-biased sex difference in neuronal cell density. The density of neurons in the SDA-DH was increased in male mice by orchidectomy on the day of birth and decreased in female mice by treatment with testosterone, dihydrotestosterone, or estradiol within 5 days after birth. These findings indicate that the SDA-DH is defeminized under the influence of testicular testosterone, which acts via both directly by binding to the androgen receptor, and indirectly by binding to the estrogen receptor after aromatization. We measured the activity of SDA-DH neurons with c-Fos, a neuronal activity marker, in female mice during maternal and sexual behaviors. The number of c-Fos-expressing neurons in the SDA-DH of female mice was negatively correlated with maternal behavior performance. However, the number of c-Fos-expressing neurons did not change during female sexual behavior. These findings suggest that the SDA-DH contains a neuronal cell population, the activity of which decreases in females exhibiting higher performance of maternal behavior, but it may contribute less to female sexual behavior. Additionally, we examined the brain of common marmosets and found an area that appears to be homologous with the mouse SDA-DH. The sexually dimorphic structure identified in this study is not specific to mice and may be found in other species.