Zidovudine (INN) or azidothymidine (AZT) (also called ZDV) is a nucleoside analog reverse-transcriptase inhibitor (NRTI), a type of antiretroviral drug used for the successful treatment of HIV/AIDS infectiousness. It is a therapeutic analog of thymidine.

AZT is the first U.S. government-approved treatment for HIV therapy, prescribed under the names Retrovir and Retrovis. AZT was the first breakthrough in AIDS therapy, significantly reducing the replication of the virus in patients and leading to clinical and immunologic improvements. It can also be used to prevent HIV transmission, such as from mother to child during the period of birth or after a needle stick. Used by itself in HIV-infected patients, AZT safely slows HIV replication in patients, but generally does not stop it entirely. This may allow HIV to become AZT-resistant over time, and for this reason AZT is usually used in conjunction with the other anti-HIV drugs in combination therapy called highly active antiretroviral therapy (HAART). To help in its use in combination, AZT is marketed used as an ingredient in Combivir and Trizivir, among others. Zidovudine is mandated in the World Health Organization's "Essential Drugs List", which is a list of minimum medical needs for a basic medical health care system.