William King Gregory (May 19, 1876 – December 29, 1970) was an American zoologist, renowned as a primatologist, paleontologist, and functional and comparative anatomist.

He developed an early interest in both fishes and the land vertebrates, publishing papers on both groups, including two in Science in 1903.

He was formally appointed to the scientific staff at the American Museum of Natural History in 1911 and became part of the teaching faculty at Columbia in 1916, eventually rising to the post of Da Costa Professor in the Department of Zoology.

His studies often had particular significance in the field of evolution; he believed the anatomical structures of fossil and extant species should exhibit relationships.

From the 1920s he became involved in the study of marsupials, in 1947 developing his palimpsest theory by showing a connection between the monotremes and early reptiles.

[5][6] He retired from the American Museum of Natural History in 1944 and from Columbia in 1945, and moved permanently to his house in Woodstock, New York, with his second wife Angela, whom he had married soon after Laura's death in 1937.