William James Sidis (; April 1, 1898 – July 17, 1944) was an American child prodigy whose exceptional abilities in mathematics and languages made him one of the most famous intellectual prodigies of the early 20th century. Born to Boris Sidis, a prominent psychiatrist, and Sarah Mandelbaum Sidis, a physician, Sidis demonstrated extraordinary intellectual capabilities from infancy. Enrolled at Harvard University at age 11, he delivered a widely publicized lecture on four-dimensional geometry at age 12 and graduated cum laude in 1914 at 16.

Despite his early academic success, Sidis deliberately withdrew from public attention following his imprisonment during the First Red Scare and spent the remainder of his life working in anonymity while pursuing private scholarly interests. His extensive writings under various pseudonyms covered topics ranging from cosmology and mathematics to Native American history and urban transportation systems. His unsuccessful privacy lawsuit against The New Yorker magazine in the 1930s established important precedents in American privacy law. Sidis has become, in the words of historian Ann Hulbert, "a cautionary tale in every debate about gifted children," representing both the potential and perils of exceptional intellectual precocity.