Oseltamivir (INN) (IPA: ) 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 new viruses 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. It is generally available by prescription only.

With increasing fears about the potential for a new influenza pandemic, oseltamivir has received substantial media attention. Production capacity is limited, and governments (and even some private individuals) are stockpiling the drug.