Williams County is located on the western border of the U.S. state of North Dakota, next to Montana. As of the 2020 census, the population was 40,950. making it the fifth-most populous county in North Dakota. Its county seat is Williston. The Williston Micropolitan Statistical Area includes all of Williams County. It is bordered on the south by the upper Missouri River, whose confluence with its tributary Yellowstone River is located just east of the border with Montana.