Thomas Parran (September 28, 1892 – February 16, 1968) was an American physician and Public Health Service officer. He was appointed the sixth Surgeon General of the United States from 1936 to 1948, and oversaw the notorious Tuskegee syphilis experiment and Guatemala syphilis experiment.

Parran was born near St. Leonard, Maryland to Benjamin and Mary (Latimer) Parran on September 28, 1892. He was raised on his family's tobacco farm. He was tutored at home by a relative and attended St. John's College in Annapolis, Maryland, on a scholarship (1911, A.B.; 1915, A.M.). Finances influenced his decision to attend Georgetown University School of Medicine (1915, M.D.) and to follow with an internship at Sibley Memorial Hospital in Washington, D.C. A lifelong interest in research was sparked during medical school. He self-identified as Thomas Parran Jr. to distinguish himself from his uncle Thomas Parran, a Maryland politician and U.S. Representative in the Sixty-second Congress (who also had a son named Thomas Parran Jr.). Later in life Parran dropped the Jr. and named his first son Thomas Parran Jr.

Parran volunteered at a health laboratory operated by the District of Columbia, under Joseph J. Kinyoun, founder of Public Health Service's Hygienic Laboratory (renamed the National Institute of Health in 1930). Kinyoun recruited Parran to join a field team of young physicians under PHS's Leslie L. Lumsden, building privies and surveying conditions in the Southern United States. In March 1917, Parran reported to Okmulgee, Oklahoma, for the first of many assignments in rural sanitation.

After receiving an Assistant Surgeon's commission in September 1917, Parran continued on assignments in rural health services administration, sanitation, and the control of communicable diseases; between field assignments, Parran tasted life as an administrator in Washington, DC. In October 1923, he joined a group of young medical officers who attended 6 months of coursework at the Hygienic Laboratory, receiving the practical equivalent of a master's degree in public health. Parran's first leadership position was as Chief of PHS's Division of Venereal Diseases (September 1926), a program begun during World War I. Parran worked to sway public sentiment away from moral condemnation of venereal diseases and toward consideration of syphilis as a medical condition and threat to public health.

His talents in rural health administration would soon lead him temporarily in a new direction. A reform-minded governor, Franklin Roosevelt, requested for Parran to be loaned to the State of New York, where in April 1930 Parran took up his post as state health commissioner. His primary task was chairing a Special Health Commission whose recommendations (1932) provided a framework to bolster county health departments in the face of needs in the Great Depression. Few of the commission's recommendations were enacted. Parran's work on syphilis achieved more success. The Columbia Broadcasting System inadvertently launched his campaign after radio executives censored the phrase "syphilis control" from a talk, leading Parran to cancel his appearance. Newspapers across the United States reprinted the censored speech.

Parran became active in New Deal politics in New York and entered national politics as well. In 1934, his former supervisor, now President of the United States, Roosevelt, appointed Parran to the Committee on Economic Security, which drafted the Social Security Act of 1935; Title VI authorized millions for public health departments and for biomedical research. After Surgeon General Hugh S. Cumming's term, President Roosevelt appointed Parran as Surgeon General of the United States; he was sworn in on 6 April 1936.

Parran's syphilis control campaign was in full swing by the fall of 1936. Title VI funds supported efforts to identify and treat syphilis, and the National Venereal Disease Control Act of 1938 made funds available for rapid treatment centers that employed the new sulfa drugs and, later, penicillin. During 1937 his book about syphilis, Shadow on the Land, was published and very well received. However, some scholars have argued that his work against syphilis is tainted by the Tuskegee Syphilis Study (1932–1972) and the Guatemala syphilis experiments (1946–1948), which were conducted by the PHS's Division of Venereal Diseases, partially during his tenure as Surgeon General.

During World War II, reports from Europe indicated that the public health situation was dire, prompt action was needed, and a highly qualified medical officer was required to direct the Public Health Division of the Civil Affairs Branch (G-5) of the newly formed headquarters. Because all of the senior Army medical officers with competence in this field were already holding highly important positions, the Army turned to Parran for help, and he released his deputy, Warren Fales Draper, to the military headquarters. Draper became a member of General Dwight Eisenhower's staff as the Chief of the Public Health Branch and the top public health adviser for Supreme Headquarters, American Expeditionary Forces with the rank of brigadier general.