Estrone (C₁₈H₂₂O₂), also known as E1 or osterone (3-hydroxy-1,3,5(10)-estratrien-17-one) is a C-18 steroid hormone. It is one of the three naturally occurring Estrogens in woman (weaker Estrogen), the two others are Estradiol and Estriol. As an Estrogen, Estrone is involved in female sexual development and function, and is one of the major hormones found in the women bodies after menopausal.

Estrone is produced through conversion of Adrostenedione primarily in the placenta, ovaries, and also in peripheral tissues (especially adipose tissue). Estrone concentrations in premenopausal mammals fluctuate according to the menstrual cycle. In humans, more than 50% of the Estrone is secreted by the ovaries. During the follicular phase of the menstrual cycle, Estrone level slightly increases with a short duration peak at around day 13, which goes down by day 16. A second peak occurs at around day 21 of the cycle and if fertilization does not occur, then the production of Estrone decreases. In prepubertal children, men and non-supplemented postmenopausal women, the major portion of Estrone is derived from peripheral tissue conversion of Androstenedione. Interconversion of Estrone and Estradiol also occurs in peripheral tissue. Androstenedione is also converted into Estrone by Aromatase (CYP19) and is expressed in stromal and carcinoma or parenchymal components of breast cancer tissue.