King William is a census-designated place (CDP) in and the county seat of King William County, Virginia, United States. The population as of the 2010 Census was 252. Located in King William is the oldest courthouse in continuous use in the United States, built in 1725. The community is also known as King William Courthouse or by an alternative spelling, King William Court House. Pamunkey Indian Tribe Museum was established in King William in 1979.