Estradiol (E2), is an estrogen steroid hormone and the major female sex hormone. It is involved in the regulation of the estrous and menstrual female reproductive cycles. Estradiol is responsible for the development of female secondary sexual characteristics such as the breasts, widening of the hips, and a feminine pattern of fat distribution in women and is important in the development and maintenance of female reproductive tissues such as the mammary glands, uterus, and vagina during puberty, adulthood, and pregnancy.It also has important effects in many other tissues including bone, fat, skin, liver, and the brain. Though estradiol levels in men are much lower compared to those in women, estradiol has important roles in men as well. Apart from humans and other mammals, estradiol is also found in most vertebrates and crustaceans, insects, fish, and other animal species.

Estradiol is produced especially within the follicles of the ovaries, but also in other tissues including the testicles, the adrenal glands, fat, liver, the breasts, and the brain. Estradiol is produced in the body from cholesterol through a series of reactions and intermediates.The major pathway involves the formation of androstenedione, which is then converted by aromatase into estrone and is subsequently converted into estradiol. Alternatively, androstenedione can be converted into testosterone, which can then be converted into estradiol. Upon menopause in women, production of estrogens by the ovaries stops and estradiol levels decrease to very low levels.

In addition to its role as a natural hormone, estradiol is used as a medication, for instance in menopausal hormone therapy, for information on estradiol as a medication, see the estradiol article.