Robert Charles Gallo (/ˈɡɑːloʊ/; born March 23, 1937) is an American biomedical researcher. He is best known for his role in establishing the human immunodeficiency virus (HIV) as the infectious agent responsible for acquired immune deficiency syndrome (AIDS) and in the development of the HIV blood test, and he has been a major contributor to subsequent HIV research.

Gallo is the director and co-founder of the Institute of Human Virology (IHV) at the University of Maryland School of Medicine in Baltimore, Maryland, established in 1996 in a partnership including the State of Maryland and the City of Baltimore. In November 2011, Gallo was named the first Homer & Martha Gudelsky Distinguished Professor in Medicine. Gallo is also a co-founder of biotechnology company Profectus BioSciences, Inc. and co-founder and scientific director of the Global Virus Network (GVN).

Gallo was the most cited scientist in the world from 1980 to 1990, according to the Institute for Scientific Information, and he was ranked third in the world for scientific impact for the period 1983–2002. He has published over 1,300 papers.

Gallo was born in Waterbury, Connecticut, to a working-class family of Italian descent. He earned a B.S. degree in biology in 1959 from Providence College and received a M.D. from Jefferson Medical College in Philadelphia, Pennsylvania, in 1963. After completing his medical residency at the University of Chicago, he became a researcher at the National Cancer Institute, where he worked for 30 years, mainly as head of the Laboratory of Tumor Cell Biology.

Gallo states that his choice of profession was influenced by the early death of his sister from leukemia, a disease to which he initially dedicated much of his research.

After listening to a talk by biologist David Baltimore and further stimulation from his virologist colleague, Robert Ting, concerning the work of the late Howard Martin Temin, Gallo became interested in the study of retroviruses, and made their study the primary activity of his lab. In 1976, Doris Morgan, a first year post-doctoral fellow in Gallo's lab, was asked by Gallo to examine culture fluid of activated lymphocytes for the possible production of growth factors. Soon she was successful in growing T lymphocytes. Gallo, Morgan and Frank Ruscetti, another researcher in Gallo's lab, coauthored a paper in Science describing their method. The Gallo group identified this as T-cell growth factor (TCGF). The name was changed in 1978 to IL-2 (interleukin-2) by the Second International Lymphokine Conference (which was held in Interlaken, Switzerland). Although earlier reports had described soluble molecules with biologic effects, the effects and biochemistry of the factors were not well characterized. One such example was the report by Julius Gordon in 1965, which described blastogenic transformation of lymphocytes in extracellular media. However, cell growth was not demonstrated and the affected cell type was not identified, making the identity of the factor(s) involved unclear and its natural function unknown.

The discovery of IL-2 allowed T cells, previously thought to be dead end cells, to be grown significantly in culture for the first time, opening research into many aspects of T cell immunology. Gallo's lab later purified and biochemically characterized IL-2. This breakthrough also allowed researchers to grow T-cells and study the viruses that affect them, such as human T-cell leukemia virus, or HTLV, the first retrovirus identified in humans, which Bernard Poiesz, another post-doctoral fellow in Gallo's lab played a key role in its isolation. HTLV's role in leukemia was clarified when Kiyoshi Takatsuki and other Japanese researchers, puzzling over an outbreak of a rare form of leukemia, later independently found the same retrovirus, and both groups showed HTLV to be the cause. At the same time, a similar HTLV-associated leukemia was identified by the Gallo group in the Caribbean. In 1982, Gallo received the Lasker Award: "For his pioneering studies that led to the discovery of the first human RNA tumor virus [the old name for retroviruses] and its association with certain leukemias and lymphomas."

On May 4, 1984, Gallo and his collaborators published a series of four papers in the scientific journal Science demonstrating that a retrovirus they had isolated, called HTLV-III in the belief that the virus was related to the leukemia viruses of Gallo's earlier work, was the cause of AIDS. A French team at the Pasteur Institute in Paris, France, led by Luc Montagnier, had published a paper in Science in 1983, describing a retrovirus they called LAV (lymphadenopathy associated virus), isolated from a patient at risk for AIDS.