Karl Leo (born 10 July 1960 in Freiburg im Breisgau, Baden-Württemberg, Germany) is a German physicist.
In 1986 he joined the Max-Planck-Institut für Festkörperforschung in Stuttgart for a PhD under the guidance of Hans Queisser.
He then joined AT&T Bell Laboratories in Holmdel (New Jersey) as a postdoctoral research associate.
In 1991 he joined the RWTH Aachen as an assistant professor and obtained the Habilitation degree.
Leo works in the field of semiconductor optics and the physics of thin organic films.