William Henry Furness III (August 10, 1866 – August 11, 1920) was an American physician, ethnographer and author from Philadelphia, Pennsylvania. He was among the first to study and photograph the Kayan people of Borneo and the Wa'ab people on the island of Yap.
He was the grandson and namesake of Unitarian theologian William Henry Furness, and the son of Shakespearean scholar Horace Howard Furness. He attended St. Paul's School (Class of 1883), Harvard University (Class of 1888), and graduated from the University of Pennsylvania Medical School in 1891. He was one of the medical students portrayed in the Thomas Eakins painting The Agnew Clinic.
Furness made four expeditions to Southeast Asia and Oceania between 1895 and 1901, accompanied by Hiram M. Hiller, Jr. and Alfred C. Harrison, Jr. The trio collected ethnographic, archaeological, and skeletal material, and Furness and Harrison took photographs. These artifacts were among the founding collections of the University of Pennsylvania Museum of Archaeology and Anthropology. Zoological specimens – mostly fish and birds – were donated to the Academy of Natural Sciences of Philadelphia. Duplicate objects were donated to the Peabody Museum of Archaeology and Ethnology at Harvard University.