Henry Havelock Ellis, known as Havelock Ellis (2 February 1859 – 8 July 1939), was an English physician, eugenicist, writer, progressive intellectual and social reformer who studied human sexuality. He co-wrote the first medical textbook in English on homosexuality in 1897, and also published works on a variety of sexual practices and inclinations, as well as on transgender psychology. He is credited with introducing the notions of narcissism and autoeroticism, later adopted by psychoanalysis.
Ellis was among the pioneering investigators of psychedelic drugs and the author of one of the first written reports to the public about an experience with mescaline, which he conducted on himself in 1896. He supported eugenics and served as president of the Eugenics Society.