William Henry Young FRS (London, 20 October 1863 – Lausanne, 7 July 1942) was an English mathematician. Young was educated at City of London School and Peterhouse, Cambridge. He worked on measure theory, Fourier series, differential calculus, amongst other fields, and made contributions to the study of functions of several complex variables. He was the husband of Grace Chisholm Young, with whom he authored and co-authored 214 papers and 4 books. Two of their children became professional mathematicians (Laurence Chisholm Young, Cecilia Rosalind Tanner). Young's Theorem was named after him.