William Herman Rau (January 19, 1855 – November 19, 1920) was an American photographer, active primarily in the latter half of the 19th and early 20th centuries. He is best remembered for his stereo cards of sites around the world, and for his panoramic photographs of sites along the Pennsylvania Railroad. He was official photographer of the St. Louis World’s Fair in 1904, and of the Lewis and Clark Centennial Exposition in Portland in 1905. His work is now included in the collections of the Smithsonian Institution and the Getty Museum.
Rau was born in Philadelphia in 1855, the son of German and Swiss immigrants Peter and Mary Witschi Rau. His older brother, George, operated a photography studio out of the Rau house, and William picked up the trade while still young. At the age of 13, he started doing photographic work for his future father-in-law, William Bell, who was a medical and survey photographer for the federal government.