Human T-cell lymphotropic virus type 1 or human T-lymphotropic virus (HTLV-I), also called the adult T-cell lymphoma virus type 1, is a retrovirus of the human T-lymphotropic virus (HTLV) family that has been implicated in several kinds of diseases including very aggressive adult T-cell lymphoma (ATL), HTLV-I-associated myelopathy, uveitis, Strongyloides stercoralis hyper-infection and some other diseases. It is thought that about 1–5% of infected persons develop cancer as a result of the infection with HTLV-I over their lifetimes.

Adult T-cell lymphoma (ATL) was discovered in 1977 in Japan. The symptoms of ATL were different from other lymphomas known at the time. It was suggested that ATL is caused by the infection of a retrovirus called ATLV. Strikingly, ATLV had the transforming activity in vitro. These studies established that the retrovirus infection is the cause of ATL. The retrovirus is now generally called HTLV-I because later studies proved that ATLV is the same as the firstly identified human retrovirus called HTLV discovered by Bernard Poiesz and Francis Ruscetti and their co-workers in the laboratory of Robert C. Gallo at the National Cancer Institute. Infection with HTLV-I, like infection with other retroviruses, probably occurs for life. A patient infected with HTLV can be diagnosed when antibodies against HTLV-1 are detected in the serum.