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.

Williston is a city in and the county seat of Williams County, North Dakota, United States. The 2020 census gave its population as 29,160, making Williston the sixth-most populous city in North Dakota. The city's population nearly doubled between 2010 and 2020, due largely to the North Dakota oil boom.