William Happer (born July 27, 1939) is an American physicist who has specialized in the study of atomic physics, optics and spectroscopy. He is the Cyrus Fogg Brackett Professor of Physics, Emeritus, at Princeton University, and a long-term member of the JASON advisory group, where he pioneered the development of adaptive optics. From 1991 to 1993, Happer served as director of the Department of Energy's Office of Science as part of the George H.W. Bush administration. He was dismissed from the Department of Energy in 1993 by the Clinton Administration after disagreements on the ozone hole.