William Gilson Farlow (December 17, 1844 – June 3, 1919) was an American botanist, born in Boston, Massachusetts, and educated at Harvard (A.B., 1866; M.D., 1870), where, after several years of European study, he became adjunct professor of botany in 1874 and professor of cryptogamic botany in 1879. Farlow corresponded with Caroline Bingham and Jacob Georg Agardh collaborating in the identification and classification of species of algae previously unknown to science. Farlow was elected to the American Academy of Arts and Sciences in 1874. In 1899 he was president of the American Society of Naturalists; in 1904 president of the National Academy of Sciences; in 1905 president of the American Association for the Advancement of Science and a member of the American Philosophical Society; and in 1911 president of the Botanical Society of America. He received honorary degrees from Harvard University, the University of Glasgow (LL.D in 1901), and the University of Wisconsin–Madison. He was known as the "father" of cryptogamic botany in the United States. Among his students was the phytologist William Albert Setchell. Among his publications are: With Charles Lewis Anderson and Daniel Cady Eaton he issued the exsiccata series Algae exsiccatae Americae Borealis (1877-1889).