Williamson is a census-designated place in Yavapai County, Arizona, United States. The population was 3,776 at the 2000 census and 6,196 at the 2020 census. The name "Wiliamson" is a misnomer, perhaps propagated by federal bureaucratic error; the area has long been known as "Williamson Valley" after the major road through the area, Williamson Valley Road.