Sir Ernest Marsden CMG CBE MC FRS (19 February 1889 – 15 December 1970) was an English-New Zealand physicist. He is recognised internationally for his contributions to science while working under Ernest Rutherford, which led to the discovery of new theories on the structure of the atom. In Marsden's later work in New Zealand, he became a significant member of the scientific community, while maintaining close links to the United Kingdom.
Born in East Lancashire, Marsden lived in Rishton and attended Queen Elizabeth's Grammar School, Blackburn, where an inter-house trophy rewarding academic excellence ('The Marsden Merit Trophy') bears his name.
In 1909, as a 20-year-old student at the University of Manchester, he met and began work under Ernest Rutherford. While still an undergraduate he conducted the famous Geiger–Marsden experiment, called the gold foil experiment, together with Hans Geiger under Rutherford's supervision. This experiment led to Rutherford's new theory for the structure of the atom, with a centralised concentration of mass and positive charge surrounded by empty space and a sea of orbiting negatively charged electrons. Rutherford later described this as "almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back to hit you".