William James Sidis (; April 1, 1898 – July 17, 1944) was an American child prodigy with mathematical and linguistic skills. He was active as a mathematician, linguist, historian, and author. He wrote the book The Animate and the Inanimate, published in 1925, in which he speculated about the origin of life in the context of thermodynamics. His works were published under various pseudonyms, as he never used his real name as an author.

His father, the psychiatrist Boris Sidis, raised his son according to certain principles with the desire for his son to be gifted. Sidis became famous first for his precocity and later for his eccentricity and withdrawal from public life. Eventually, he avoided mathematics altogether, writing on other subjects under a number of pseudonyms. He entered Harvard University at age 11 and, as an adult, was claimed by family members to have an IQ between 250 and 300, and to be conversant in about 25 languages and dialects. These statements have not been verified, but many of his contemporaries, including Norbert Wiener and Daniel Frost Comstock, agreed that he was extremely intelligent.