William D. Sill (1937-2008) was a geologist, professor, and religious educator in Argentina and the United States.

William Sill was born July 30, 1937 in Las Vegas, Nevada to James M. and Louise Sill. He learned to love geology as a child, worked hard, and graduated high school in 1955. He graduated from Brigham Young University in geology in 1963, having taken some time to serve his country in the Coast Guard and Army Reserves (1956-1957). Sill also served a mission for the Church of Jesus Christ of Latter-day Saints to Argentina from 1958 to 1961. Three months later, he and Nelida, an acquaintance from his mission, were married; the couple have since been sealed in the temple and are the parents of four children. Prior to graduation from BYU, Sill was accepted into Harvard. While at Harvard, he participated in paleontology digs in Kenya, 1964-5, and 1967. Sill received his Ph.D. in 1968 in geology, with a focus on vertebrate paleontology.

Following his graduation he was granted a postdoctoral fellowship working as a research associate at Yale University. In 1971 he became a professor of paleontology at the Universidad Nacional de San Juan in Argentina, and served as chair of the Department of Geology from 1973 to 1974. He then returned to accepted the position of director of the Church Education System in Argentina, before returning to the United States to work as an Institute of Religion director in Austin, Texas and Norman, Oklahoma.

In 1992 he moved again to Argentina to become the curator paleontology at the museum in San Juan. There he led geological expeditions in Ischigualisto, Argentina, a site he had worked to have declared a national park in 1973. During these years he worked to have the park named a UNESCO World Heritage Site in 2000. Sill returned to Las Vegas for retirement in 2002. He passed away on March 15, 2008.