An antiestrogen is a substance that blocks the production or utilization of estrogens, or inhibits their effects. (Estrogens are the family of hormones that promote the development and maintenance of female sex characteristics.)

Antiestrogens like tamoxifen can promote an invasive phenotype in estrogen receptor (ER)-positive breast cancer cells with deficient intercellular adhesion.

Although aromatase inhibitors could be considered antiestrogens by some definitions, they are often considered to be a distinct class. Aromatase inhibitors reduce the production of estrogen, while the term "antiestrogen" is usually reserved for agents reducing the response to estrogen.