Austrian-born physicist who was a joint recipient, with Carl D. Anderson of the United States, of the Nobel Prize for Physics in 1936 for his discovery of cosmic rays (cosmic ray)—high-energy radiation originating in outer space.

Educated at the University of Graz, Hess received his Ph.D. from the University of Vienna in 1906. His research dealt chiefly with radioactivity and atmospheric electricity. For many years scientists had been unable to explain the source of an ionizing background radiation in the atmosphere that penetrated electroscopes sent aloft in balloons. It was assumed that the radiation must have its source on Earth, but preliminary findings suggesting that the radiation increased when measured at higher points above the Earth's surface cast doubt upon this hypothesis. In a series of balloon ascents in 1911–13, Hess found that the radiation increased rapidly with altitude, and suggested it had extraterrestrial origins. In 1925, Hess's theory was confirmed by Robert Andrews Millikan, who gave the radiation the name of cosmic rays. Cosmic-ray research soon emerged as an important branch of physics and led to the discovery of several new fundamental particles—including the positron, discovered by Anderson in 1932—as well as advances in astrophysics and cosmology.

Hess taught and conducted research at the universities of Vienna (1910–20), Graz (1920–31), and Innsbruck (1931–37). He left Austria in 1937 to escape the Nazis and settled in the United States, where he taught at Fordham University in New York City until 1956.