William David Barnes (September 3, 1860 – May 1, 1930, Decatur, Illinois) was an American entomologist and surgeon.

[1] While at Harvard, he met naturalist Louis Agassiz and his love of Lepidoptera grew.

He completed an internship at Boston City Hospital and then studied abroad in Heidelberg, Munich, and Vienna.

From 1910 to 1919 James Halliday McDunnough produced, with Barnes credited as co-author, an impressive volume of research on the taxonomy of North American Lepidoptera, including the first four volumes of the privately published Contributions to the Natural History of the Lepidoptera of North America, the 1917 Check List of the Lepidoptera of Boreal America, Illustrations of the North American Species of the Genus Catocala, and numerous journal articles, 67 papers in all.

[2] At his death in 1930, his collection of butterflies was regarded as the largest and finest in the world with possibly 10,000 species and an estimated 473,000 specimens, even receiving praise in the 1936 National Geographic magazine.