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Ralph Walter Graystone Wyckoff (born August 9, 1897 in Geneva, New York; died November 3, 1994 in Tucson, Arizona) was an American scientist and pioneer of X-ray crystallography.

He was the son of judge Abram Ralph and Ethel Agnes (n�e Catchpole) Wyckoff. He studied at Hobart College, where he made bachelor of science, continued at Cornell University, and published his first scientific paper (of more than 400) at the age of nineteen. Under Shoji Nishikawa, he presented his thesis about the crystallographic resolution of the structures of NaNO₃ and CsICl₂ in 1919.

He continued working in X-ray crystallography and wrote several books about the topic. He moved to the Rockefeller University, which he left again in 1937 to take up studies of bacteria and, especially, viruses. In Ann Arbor, he invented a technique to take three-dimensional electron microscope images of bacteria. From 1946 to 1952, he researched macromolecules and viruses at the National Institutes of Health in Bethesda. Later appalled by growing buerocracy at the NIH, he took the job of professor of microbiology and physics at the University of Arizona in Tucson, where he was forced to retire at the age of 80.

Wyckoff was married two times, the first time short and without children, the second marriage resulted in three daughters.