Lentivirus is a genus of retroviruses that cause chronic and deadly diseases characterized by long incubation periods, in humans and other mammalian species. The genus includes the human immunodeficiency virus (HIV), which causes AIDS. Lentiviruses are distributed worldwide, and are known to be hosted in apes, cows, goats, horses, cats, and sheep as well as several other mammals.

Lentiviruses can integrate a significant amount of viral complementary DNA into the DNA of the host cell and can efficiently infect nondividing cells, so they are one of the most efficient methods of gene delivery. They can become endogenous, integrating their genome into the host germline genome, so that the virus is henceforth inherited by the host's descendants.

Five serogroups of lentiviruses are recognized, reflecting the vertebrate hosts with which they are associated (primates, sheep and goats, horses, domestic cats, and cattle). The primate lentiviruses are distinguished by the use of CD4 protein as a receptor and the absence of dUTPase.

The genus contains the following species, listed by scientific name and followed by the exemplar virus of the species:

The virions are enveloped viruses 80–100 nm in diameter. They are spherical or pleomorphic, with capsid cores that mature to a cylindrical or conical shape. Projections of envelope make the surface appear rough, or tiny spikes (about 8 nm) may be dispersed evenly over the surface.

Lentiviruses contain 2 positive sense, single-strand RNAs that are bound by nucleocapsid proteins. As with all retroviruses, lentiviruses have gag, pol and env genes, coding for viral proteins in the order: 5´-gag-pol-env-3´. Unlike other retroviruses, however, lentiviruses have two regulatory genes, tat and rev. They may also have additional accessory genes depending on the virus (e.g., for HIV-1: vif, vpr, vpu, nef) whose products are involved in regulation of synthesis and processing viral RNA and other replicative functions. The long terminal repeat (LTR) is about 600 nt long, of which the U3 region is 450, the R sequence 100 and the U5 region some 70 nt long.

Retroviruses carry proteins within their capsids, which bind the RNA genome. These proteins are typically involved in the early stages of genome replication, and include reverse transcriptase and integrase. Reverse transcriptase is the virally encoded RNA-dependent DNA polymerase. The enzyme uses the viral RNA genome as a template for the synthesis of a complementary DNA copy. Reverse transcriptase possesses [RNase H] activity for destruction of the RNA-template. Integrase binds both the viral cDNA generated by reverse transcriptase and the host DNA. It then processes the LTRs before inserting the viral genome into the host DNA. Tat acts as a trans-activator during transcription to enhance initiation and elongation. The Rev responsive element acts post-transcriptionally, regulating mRNA splicing and transport to the cytoplasm.

The lentiviral proteome consists of five major structural proteins and three or four non-structural proteins. Primate lentiviruses lack the dUTPase, so they have three non-structural proteins, whereas all other lentiviruses encode it.