Ian Pretyman Stevenson (October 31, 1918 – February 8, 2007) was a Canadian-born American psychiatrist, the founder and director of the Division of Perceptual Studies at the University of Virginia School of Medicine. He was a professor at the University of Virginia School of Medicine for fifty years. He was chair of their department of psychiatry from 1957 to 1967, Carlson Professor of Psychiatry from 1967 to 2001, and Research Professor of Psychiatry from 2002 until his death in 2007. He helped to found the Society for Scientific Exploration in 1982.

He is best known for his research into evidence of reincarnation – the premise that emotions, memories, and even physical bodily features can be passed on from one life to another. Over his forty years of international research, he amassed three thousand cases of children who claimed to remember past lives. Stevenson was the author of around three hundred papers and fourteen books on reincarnation, including Twenty Cases Suggestive of Reincarnation (1966), Reincarnation and Biology and its simplified version Where Reincarnation and Biology Intersect (both 1997), and European Cases of the Reincarnation Type (2003).

Stevenson was cautious in making claims about reincarnation. He emphasized that the information he collected was suggestive of reincarnation but "was not flawless and it certainly does not compel such a belief." He did, however, believe he had produced a body of evidence for reincarnation that should be taken seriously. His position was that reincarnation might possibly represent a third contributing factor, in addition to genetics and the environment, in the development of certain phobias, philias, unusual abilities, and illnesses.

In an obituary for Stevenson in The New York Times, Margalit Fox wrote that Stevenson's supporters saw him as a misunderstood genius, that his detractors regarded him as earnest but gullible, but that most scientists had simply ignored his research. Stevenson's critics contend that ultimately his conclusions are undermined by confirmation bias and motivated reasoning, and were reliant on anecdotal evidence rather than controlled experimental work. His case reports were also criticized for containing errors and omissions. Upon his retirement, Stevenson's work was continued by research colleagues such as Jim B. Tucker, Antonia Mills, Satwant Pasricha, and Erlendur Haraldsson.

Ian Stevenson was born in Montreal and raised in Ottawa and was one of three children. His father, John Stevenson, was a Scottish lawyer who was working in Ottawa as the Canadian correspondent for The Times of London or The New York Times. His mother, Ruth, had an interest in theosophy and an extensive library on the subject, to which Stevenson attributed his own early interest in the paranormal. As a child he was often bedridden with bronchitis, a condition that continued into adulthood and engendered in him a lifelong love of books. According to Emily Williams Kelly, a colleague of his at the University of Virginia, he maintained a list of the books he had read, which numbered 3,535 between 1935 and 2003.

He studied medicine at St. Andrews University in Scotland from 1937 to 1939, but had to complete his studies in Canada because of the outbreak of the Second World War. He graduated from McGill University with a B.S.c. in 1942 and an M.D. in 1943. He was married to Octavia Reynolds from 1947 until her death in 1983. In 1985, he married Dr. Margaret Pertzoff (1926–2009), professor of history at Randolph-Macon Woman's College. She did not share his views on the paranormal, but tolerated them with what Stevenson called "benevolent silences."

After graduating, Stevenson conducted research in biochemistry. His first residency was at the Royal Victoria Hospital in Montreal (1944–1945), but his lung condition continued to bother him, and one of his professors at McGill advised him to move to Arizona for his health. He took up a residency at St. Joseph's Hospital in Phoenix, Arizona (1945–1946). After that, he held a fellowship in internal medicine at the Alton Ochsner Medical Foundation in New Orleans, became a Denis Fellow in Biochemistry at Tulane University School of Medicine (1946–1947), and a Commonwealth Fund Fellow in Medicine at Cornell University Medical College and New York Hospital (1947–1949). He became a U.S. citizen in 1949.

Emily Williams Kelly writes that Stevenson became dissatisfied with the reductionism he encountered in biochemistry, and wanted to study the whole person. He became interested in psychosomatic medicine, psychiatry and psychoanalysis, and in the late 1940s, worked at New York Hospital exploring psychosomatic illness and the effects of stress, and in particular why, for example, one person's response to stress might be asthma and another's high blood pressure.