Arthur Llewelyn Hughes (December 18, 1883 – June 25, 1978) was an British physicist born in Liverpool. He studied physics at Liverpool University under Charles Barkla, and later worked at Cavendish Laboratory at Cambridge under J. J. Thomson. It was at Cambridge that Hughes carried out what some consider his finest scientific work, proving the validity of Einstein's theory of the photoelectric effect.

In 1912, he accepted a teaching position at the newly-founded Rice Institute in Texas. When World War I broke out, Hughes returned to England to work on anti-submarine devices. Shortly after the war he left Rice to take a research professorship at Queen's College in Ontario, from which he was called to St. Louis by Arthur Holly Compton in 1923. Hughes served as head of the physics department until 1952.

He worked on the Manhattan Project in St. Louis on the cyclotron from 1939-1945 and plutonium production and at Los Alamos, New Mexico during World War II. He served as an assistant director at Los Alamos atomic energy project, acting as recruiting officer.

Hughes' own interests in physics have been broad: the list of his publications, for instance, shows a steady output of articles from 1910 into the 1960s. His Photoelectric Phenomena, published in 1933 with Lee A. DuBridge, remained the standard work on the subject until 1959, when it went out of print. Since a physics text can become obsolete in a very short time, the endurance of this work was remarkable.