David Reimer (born Bruce Peter Reimer; 22 August 1965 – 4 May 2004) was a Canadian boy raised as a girl following medical advice and intervention after his penis was severely injured during a botched circumcision in infancy.

The psychologist John Money oversaw the case and incorrectly reported the reassignment as successful and as evidence that gender identity is primarily learned. The academic sexologist Milton Diamond later reported that Reimer's realization that he was not a girl occurred between the ages of 9 and 11 years and that he was living as a male by the age of 15. Well known in medical circles for years anonymously as the "John/Joan" case, Reimer later went public with his story to help discourage similar medical practices. He killed himself at age 38, two days after separating from his wife.