Thiomersal (INN) (C 9 H 9 HgNaO 2 S), commonly known in the United States as thimerosal, is an organomercury compound (approximately 49% mercury by weight) used as an antiseptic and antifungal agent.

It was developed and registered under the trade name Merthiolate in 1928 by the pharmaceutical company Eli Lilly and Company, and has been used as a preservative in vaccines, immunoglobulin preparations, skin test antigens, antivenins, ophthalmic and nasal products, and tattoo inks.

In the U.S., the European Union, and a few other affluent countries, the compound is being phased out from vaccines routinely given to children. Packaging the vaccines in single-dose vials eliminates the need for bacteriostatics such as thiomersal.