Robert Sidney Cahn (9 June 1899 – 15 June 1981) was a British chemist, best known for his contributions to chemical nomenclature and stereochemistry, particularly by the Cahn–Ingold–Prelog priority rules, which he proposed in 1956 with Christopher Kelk Ingold and Vladimir Prelog. Cahn was the first to report the structure of Cannabinol (CBN) found in Cannabis in the early 1930s.
Cahn was born in Hampstead, London. He became a fellow of the Royal Institute of Chemistry and was editor of the Journal of the Chemical Society from 1949 until 1963, and he remained with the Society as Director of Publications Research until his retirement in 1965.