Lentivirus (lente-, Latin for "slow") is a genus of viruses of the Retroviridae family, characterized by a long incubation period. Lentiviruses can deliver a significant amount of genetic information into the DNA of the host cell and have the unique ability among retroviruses of being able to replicate in non-dividing cells, so they are one of the most efficient methods of a gene delivery vector. HIV, SIV, and FIV are all examples of lentiviruses.

Five serogroups of lentiviruses are recognized, reflecting the vertebrate hosts with which they are associated (primates, sheep and goats, horses, cats, and cattle). The primate lentiviruses are distinguished by the use of CD4 protein as receptor and the absence of dUTPase. Some groups have cross-reactive gag antigens (e.g., the ovine, caprine and feline lentiviruses). Antibodies to gag antigens in lions and other large felids indicate the existence of other viruses related to FIV and the ovine/caprine lentiviruses. Description is on taxonomic level of genus.