Rosalyn Sussman Yalow (born on July 19, 1921) is an American medical physicist, and a co-winner of the 1977 Nobel Prize in Physiology or Medicine for her development of the radioimmunoassay (RIA) technique.

Born Rosalyn Sussman in New York City, she graduated in 1941 from Hunter College, where she developed an interest in physics. Not believing that any good graduate school would admit and provide financial support to a woman (and a Jewish woman, at that), Sussman took a job as a secretary to a leading biochemist. However, soon after graduating she received an offer for a teaching assistantship in physics from the University of Illinois because World War II came up and all the men went off to war and the university offered scholarships for women rather than close down. She was the only woman among the department's 400 members, and the first since 1917. She married fellow student Aaron Yalow in 1943, and received her Ph.D. in 1945.

After graduating, Yalow joined the Bronx Veterans Administration Hospital to help set up its radioisotope service. There she collaborated with Solomon Berson to develop RIA, a radioisotope tracing technique that allows the measurement of tiny quantities of various biological substances in the blood. Originally used to study insulin levels in diabetes mellitus, the technique has since been applied to hundreds of other substances – including hormones, vitamins and enzymes – all previously too small to detect. Despite its huge commercial potential, Yalow and Berson refused to patent the method.

In 1976 Yalow became the first female recipient of the Albert Lasker Award for Basic Medical Research. The following year she received the Nobel Prize, together with Roger Guillemin and Andrew V. Schally. Berson had died in 1972, and so could not share either prize.