Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). HGT is an important factor in the evolution of many organisms. HGT is influencing scientific understanding of higher-order evolution while more significantly shifting perspectives on bacterial evolution.

Horizontal gene transfer is the primary mechanism for the spread of antibiotic resistance in bacteria, and plays an important role in the evolution of bacteria that can degrade novel compounds such as human-created pesticides and in the evolution, maintenance, and transmission of virulence. It often involves temperate bacteriophages and plasmids. Genes responsible for antibiotic resistance in one species of bacteria can be transferred to another species of bacteria through various mechanisms of HGT such as transformation, transduction and conjugation, subsequently arming the antibiotic resistant genes' recipient against antibiotics. The rapid spread of antibiotic resistance genes in this manner is becoming a challenge to manage in the field of medicine. Ecological factors may also play a role in the HGT of antibiotic resistant genes.

Horizontal gene transfer is recognized as a pervasive evolutionary process that distributes genes between divergent prokaryotic lineages and can also involve eukaryotes. HGT events are thought to occur less frequently in eukaryotes than in prokaryotes. However, growing evidence indicates that HGT is relatively common among many eukaryotic species and can have an impact on adaptation to novel environments. Its study, however, is hindered by the complexity of eukaryotic genomes and the abundance of repeat-rich regions, which complicate the accurate identification and characterization of transferred genes.

It is postulated that HGT promotes the maintenance of a universal life biochemistry and, subsequently, the universality of the genetic code.

Griffith's experiment, reported in 1928 by Frederick Griffith, was the first experiment suggesting that bacteria are capable of transferring genetic information through a process known as transformation. Griffith's findings were followed by research in the late 1930s and early 1940s that isolated DNA as the material that communicated this genetic information.

Horizontal genetic transfer was then described in Seattle in 1951, in a paper demonstrating that the transfer of a viral gene into Corynebacterium diphtheriae created a virulent strain from a non-virulent strain, simultaneously revealing the mechanism of diphtheria (that patients could be infected with the bacteria but not have any symptoms, and then suddenly convert later or never), and giving the first example for the relevance of the lysogenic cycle. Inter-bacterial gene transfer was first described in Japan in a 1959 publication that demonstrated the transfer of antibiotic resistance between different species of bacteria. In the mid-1980s, Syvanen postulated that biologically significant lateral gene transfer has existed since the beginning of life on Earth and has been involved in shaping all of evolutionary history.

As Jian, Rivera and Lake (1999) put it: "Increasingly, studies of genes and genomes are indicating that considerable horizontal transfer has occurred between prokaryotes" (see also Lake and Rivera, 2007). The phenomenon appears to have had some significance for unicellular eukaryotes as well. As Bapteste et al. (2005) observe, "additional evidence suggests that gene transfer might also be an important evolutionary mechanism in protist evolution."

Grafting of one plant to another can transfer chloroplasts (organelles in plant cells that conduct photosynthesis), mitochondrial DNA, and the entire cell nucleus containing the genome to potentially make a new species. Some Lepidoptera (e.g. monarch butterflies and silkworms) have been genetically modified by horizontal gene transfer from the wasp bracovirus. Bites from insects in the family Reduviidae (assassin bugs) can, via a parasite, infect humans with the trypanosomal Chagas disease, which can insert its DNA into the human genome. It has been suggested that lateral gene transfer to humans from bacteria may play a role in cancer.