Varmus, Harold Eliot (1939-) is an American physician and cancer researcher who shared the 1989 Nobel Prize in physiology or medicine with his colleague J. Michael Bishop. They discovered that normal growth-regulating genes can mutate and cause cancerous growth.

Varmus was born in Oceanside, New York, and graduated from Amherst College with a bachelor's degree in 1961. He received his master's degree from Harvard University in 1962 and his medical degree from Columbia University in 1966. From 1968 to 1970, he was a clinical associate at the National Institutes of Health (NIH) in Bethesda, Maryland. In 1971, Varmus went to the University of California, San Francisco (UCSF), as a doctoral fellow, and in 1974, he joined the faculty. He became full professor of microbiology, biochemistry, and biophysics in 1979.

In the mid-1970's, Varmus and Bishop researched the genetic makeup of cancer cells. They studied the Rous sarcoma, a virus that caused tumors in chickens by attaching to a normal gene as it duplicated within a cell, and discovered that the virus's cancer-causing gene, called an oncogene, is actually a modified copy of a normal gene, called a proto-oncogene. Their research proved valuable in identifying additional proto-oncogenes that can turn dangerous and in establishing that factors other than viruses, such as toxic chemicals and radiation, may trigger such changes.

From the late 1970's through the early 1990's, Varmus researched the behavior of retroviruses such as HIV, which causes AIDS, and the hepatitis B virus. In 1993, he was appointed director of NIH. He remained there until 1999, when he became president of the Memorial Sloan-Kettering Cancer Center, a private research and treatment facility in New York City.