Oseltamivir (INN) ( ) is an antiviral drug that is used in the treatment and prophylaxis of both Influenzavirus A and Influenzavirus B. Like zanamivir, oseltamivir is a neuraminidase inhibitor. It acts as a transition-state analogue inhibitor of influenza neuraminidase, preventing progeny virions from emerging from infected cells.

Oseltamivir was the first orally active neuraminidase inhibitor commercially developed. It is a prodrug, which is hydrolysed hepatically to the active metabolite, the free carboxylate of oseltamivir (GS4071). It was developed by Gilead Sciences and is currently marketed by Hoffmann-La Roche (Roche) under the trade name Tamiflu. In Japan, it is marketed by Chugai Pharmaceutical Co., which is more than 50% owned by Roche. Oseltamivir is generally available by prescription only.

Roche estimates that 50 million people have been treated with oseltamivir. The majority of these have been in Japan, where an estimated 35 million have been treated.

With increasing fears about the potential for a new influenza pandemic, oseltamivir has received substantial media attention. Governments, corporations, and even some private individuals are stockpiling the drug. Production is currently sufficient to meet the demand for seasonal influenza and for government stockpiling. It is possible that shortages could re-emerge in the event of an actual influenza pandemic.