A viral vector is a modified virus designed to deliver genetic material into cells. This process can be performed inside an organism or in cell culture. Viral vectors have widespread applications in basic research, agriculture, and medicine.

Viruses have evolved specialized molecular mechanisms to transport their genomes into infected hosts, a process termed transduction. This capability has been exploited for use as viral vectors, which may integrate their genetic cargo—the transgene—into the host genome, although non-integrative vectors are also commonly used. In addition to agriculture and laboratory research, viral vectors are widely applied in gene therapy: as of 2022, all approved gene therapies were viral vector-based. Further, compared to traditional vaccines, the intracellular antigen expression enabled by viral vector vaccines offers more robust immune activation.

Many types of viruses have been developed into viral vector platforms, ranging from retroviruses to cytomegaloviruses. Different viral vector classes vary widely in strengths and limitations, suiting some to specific applications. For instance, relatively non-immunogenic and integrative vectors like lentiviral vectors are commonly employed for gene therapy. Chimeric viral vectors—such as hybrid vectors with qualities of both bacteriophages and eukaryotic viruses—have also been developed.

Viral vectors were first created in 1972 by Paul Berg. Further development was temporarily halted by a recombinant DNA research moratorium following the Asilomar Conference and stringent National Institutes of Health regulations. Once lifted, the 1980s saw both the first recombinant viral vector gene therapy and the first viral vector vaccine. Although the 1990s saw significant advances in viral vectors, clinical trials had a number of setbacks, culminating in Jesse Gelsinger's death. However, in the 21st century, viral vectors experienced a resurgence and have been globally approved for the treatment of various diseases. They have been administered to billions of patients, notably during the COVID-19 pandemic.

Viruses, infectious agents composed of a protein coat that encloses a genome, are the most numerous biological entities on Earth. As they cannot replicate independently, they must infect cells and hijack the host's replication machinery in order to produce copies of themselves. Viruses do this by inserting their genome—which can be DNA or RNA, either single-stranded or double-stranded—into the host. Some viruses may integrate their genome directly into that of the host in the form of a provirus.

This ability to transfer foreign genetic material has been exploited by genetic engineers to create viral vectors, which can transduce the desired transgene into a target cell. Viral vectors consists of three components:

Viral vectors are routinely used in a basic research setting and can introduce genes encoding, for instance, complementary DNA, short hairpin RNA, or CRISPR/Cas9 systems for gene editing. Viral vectors are employed for cellular reprogramming, like inducing pluripotent stem cells or differentiating adult somatic cells into different cell types. Researchers also use viral vectors to create transgenic mice and rats for experiments. Viral vectors can be used for in vivo imaging via the introduction of a reporter gene. Further, transduction of stem cells can permit the tracing of cell lineage during development.

Gene therapy seeks to modulate or otherwise affect gene expression via the introduction of a therapeutic transgene. Gene therapy by viral vectors can be performed by in vivo delivery by directly administering the vector to the patient, or ex vivo by extracting cells from the patient, transducing them, and then reintroducing the modified cells into the patient. Viral vector gene therapies may also be used for plants, tentatively enhancing crop performance or promoting sustainable production.