William Brewster (July 5, 1851 - 1919) was an American ornithologist.

Brewster was the curator of birds at the Museum of Comparative Zoology at Harvard University from 1886 to 1900. He was the co-founder, with Elliott Coues and Joel Asaph Allen, of the American Ornithologists' Union in 1883. He wrote Birds of the Cape Regions of Lower California (1902) and Birds of the Cambridge Region of Massachusetts (1906).