HIV/AIDS denialism is the belief, despite conclusive evidence to the contrary, that the human immunodeficiency virus (HIV) does not cause acquired immune deficiency syndrome (AIDS). Some of its proponents reject the existence of HIV, while others accept that HIV exists but argue that it is a harmless passenger virus and not the cause of AIDS. Insofar as they acknowledge AIDS as a real disease, they attribute it to some combination of sexual behavior, recreational drugs, malnutrition, poor sanitation, haemophilia, or the effects of the medications used to treat HIV infection (antiretrovirals).

The scientific consensus is that the evidence showing HIV to be the cause of AIDS is conclusive and that HIV/AIDS denialist claims are pseudoscience based on conspiracy theories, faulty reasoning, cherry picking, and misrepresentation of mainly outdated scientific data. With the rejection of these arguments by the scientific community, HIV/AIDS denialist material is now targeted at less scientifically sophisticated audiences and spread mainly through the Internet.

Despite its lack of scientific acceptance, HIV/AIDS denialism has had a significant political impact, especially in South Africa under the presidency of Thabo Mbeki. Scientists and physicians have raised alarm at the human cost of HIV/AIDS denialism, which discourages HIV-positive people from using proven treatments. Public health researchers have attributed 330,000 to 340,000 AIDS-related deaths, along with 171,000 other HIV infections and 35,000 infant HIV infections, to the South African government's former embrace of HIV/AIDS denialism. The interrupted use of antiretroviral treatments is also a major global concern as it potentially increases the likelihood of the emergence of antiretroviral-resistant strains of the virus.