Tamoxifen is an oral selective estrogen receptor modulator which is used in breast cancer treatment, and is currently the world's largest selling breast cancer treatment. It is used for the treatment of early and advanced breast cancer in pre- and post-menopausal women. It is also approved by the FDA for the reduction of the incidence of breast cancer in women at high risk of developing the disease. It has been further approved for the reduction of contralateral (in the opposite breast) breast cancer. Tamoxifen is sometimes used to treat gynecomastia in men. A rare condition occasionally treated with tamoxifen is retroperitoneal fibrosis.

Tamoxifen was invented by ICI Pharmaceuticals (now AstraZeneca) and is sold under the brand names Nolvadex, Istubal, and Valodex. It is also available as a generic drug in a number of countries. In the United States, the drug was almost always referred by its generic name, even before patent expiration.