William Robert Sickinger (November 7, 1926 – May 9, 2013) was an American theater director, based in Chicago. He was regarded as the founder of "off-Loop" theater, the Chicago equivalent of Off-Broadway. He often cast non-professional actors in his productions, and in doing so occasionally found new talent such as David Mamet, Marilu Henner, Jim Jacobs, and Mike Nussbaum, all of whom were Sickinger proteges.