Robert Eugene Williams (October 14, 1940) is an American astronomer who served as the director of the Space Telescope Science Institute (STScI) from 1993 to 1998, and the president of the International Astronomical Union (IAU) from 2009 to 2012. Prior to his work at STScI, he was a professor of astronomy at the University of Arizona in Tucson for 18 years and the director of Cerro Tololo Inter-American Observatory from 1986 to 1993.