Sir Alexander Haddow FRS FRSE (18 January 1907 – 21 January 1976) was a Scottish physician and pathologist at the forefront of cancer research in the 1940s. He served as Director of the Institute of Cancer Research from 1946 to 1969. He was also President of the Universal Union Against Cancer. His most important discovery was the Haddow Effect, by which a carcinogenic compound can be used to arrest a cancer whose origin is an unrelated carcinogen.