John Marion Thoday FRS (30 August 1916 – 25 August 2008) was a British geneticist. He was the son of the botanist David Thoday. He was Arthur Balfour Professor of Genetics at Cambridge University between 1959 and 1983 and was elected a Fellow of the Royal Society in 1965. Thoday was born in Chinley, Derbyshire, and educated at Bootham School, York, followed by University College of North Wales at Bangor, and then Trinity College, Cambridge. During World War II he served in the RAF as a photographic intelligence officer.