William Louis Nack (February 4, 1941 – April 13, 2018) was an American journalist and author. He wrote about sports, politics and the environment at Newsday for 11 years before joining the staff of Sports Illustrated in 1978 as an investigative reporter and general feature writer. After leaving S.I. in 2001, Nack freelanced for numerous publications, including GQ and ESPN.com. He also served as an adviser on the made-for-TV-movie Ruffian (2007) and the Disney feature Secretariat (2010).