Thimerosal is an organometallic compound containing mercury, commonly used as a preservative. It is found in cosmetics, including makeup removers, eye moisturizers, and mascaras. Some other places in which Thimerosal may be found include: soap-free cleansers; nose, eye, and ear drops; eye ointments; topical medications; and antiseptic sprays. It is also found in cleaning fluids that are used for contact lenses. Thimerosal is also used widely as a preservative in antitoxins, tuberculin tests, desensitization solutions and vaccines, most controversially including those widely administered to children. Its chemical formula is C₉H₉HgNaO₂S. It has been in use since the 1930s.

Other names for thimerosal:
Thimerosol, Merthiolate, Sodium ethylmercurithiosalicylate, Mercurothiolate, Thiomersal, Thiomersalate, Thiomersalan, Ethyl (2-mercaptobenzoato-S) mercury sodium salt, [(o-carboxyphenyl)thio] Ethylmercury sodium salt