Harold Eliot Varmus (born December 18, 1939) is an American Nobel Prize-winning scientist. He is currently the Lewis Thomas University Professor of Medicine at Weill Cornell Medicine and a senior associate at the New York Genome Center.

He was a co-recipient (along with J. Michael Bishop) of the 1989 Nobel Prize in Physiology or Medicine for discovery of the cellular origin of retroviral oncogenes. He was also the director of the National Institutes of Health from 1993 to 1999 and the 14th Director of the National Cancer Institute from 2010 to 2015, a post to which he was appointed by President Barack Obama.

Varmus was born on December 18, 1939, to Beatrice, a social service worker, and Frank Varmus, a physician, Jewish parents of Eastern European descent, in Oceanside, New York. In 1957, he graduated from Freeport High School in Freeport, New York, and enrolled at Amherst College, intending to follow in his father's footsteps as a medical doctor, but eventually graduating with a B.A. in English literature. He went on to earn an M.A. in English at Harvard University in 1962, before changing his mind once again and applying to medical schools. He was twice rejected from Harvard Medical School. That same year, he entered the Columbia University College of Physicians and Surgeons and later worked at a missionary hospital in Bareilly, India, and the Columbia Presbyterian Medical Center. As an alternative to serving militarily in the Vietnam War, Varmus joined the Public Health Service at the National Institutes of Health in 1968. Working under Ira Pastan, he researched the regulation of bacterial gene expression by cyclic AMP. In 1970, he began postdoctoral research in Bishop's lab at University of California, San Francisco.

To fulfill his national service obligations during the Vietnam War, Varmus became a member of the commissioned corps of the Public Health Service, working as a Clinical Associate in the laboratory of Ira Pastan at the National Institutes of Health from 1968 to 1970. During this first period of laboratory research, he and Pastan and their colleagues described aspects of the mechanism by which the lac operon of E. coli is regulated transcriptionally by cyclic AMP. In 1970, he and his wife, Constance Casey, moved to San Francisco, where he began post-doctoral studies with Michael Bishop at University of California, San Francisco under a fellowship from the California Division of the American Cancer Society. Appointed as an assistant professor in the UCSF Department of Microbiology and Immunology in 1972, he was promoted to professor in 1979 and became an American Cancer Society Research Professor in 1984.

During the course of his years at UCSF (1970 to 1993), Varmus's scientific work was focused principally on the mechanisms by which retroviruses replicate, cause cancers in animals, and produce cancer-like changes in cultured cells. Much of this work was conducted jointly with Michael Bishop in a notably long scientific partnership. Their best-known accomplishment was the identification of a cellular gene (c-Src) that gave rise to the v-Src oncogene of Rous sarcoma virus, a cancer-causing virus first isolated from a chicken sarcoma by Peyton Rous in 1910. Their discovery triggered the identification of many other cellular proto-oncogenes—progenitors of viral oncogenes and targets for mutations that drive human cancers. Much of this work and its consequences are described in his Nobel lecture and Bishop's, in Varmus's book The Art and Politics of Science, and in numerous histories of cancer research.

Other significant components of Varmus's scientific work over the past four and a half decades include descriptions of the mechanisms by which retroviral DNA is synthesized and integrated into chromosomes; discovery of the Proto-oncogene Wnt-1 with Roel Nusse; elucidation of aspects of the replication cycle of hepatitis B virus (with Donald Ganem); discovery of ribosomal frameshifting to make retroviral proteins (with Tyler Jacks); isolation of a cellular receptor for avian retroviruses (with John Young and Paul Bates); characterization of mutations of the epidermal growth factor receptor gene in human lung cancers, including a common mutation that confers drug resistance (with William Pao); and generation of numerous mouse models of human cancer. Notably, Varmus continued to conduct or direct laboratory work throughout his service in leadership positions at the NIH, MSKCC, and NCI.

In the early 1990s, following the award of their Nobel Prize, Varmus and Bishop became active in the politics of science, working principally with UCSF colleagues Bruce Alberts and Marc Kirschner, and with the Joint Steering Committee (later renamed the Coalition for the Life Sciences). He also co-chaired Scientists and Engineers for Clinton-Gore during the 1992 presidential campaign.

In 2025, Dr. Varmus was among 75 Nobel Laureates that advocated that Robert Kennedy Jr. not be confirmed as Secretary of Health and Human Services due to concerns on the public health impact of the nomination given Mr. Kennedy's previous comments on vaccines and lack of health policy experiences.