Cass County is a county in the U.S. state of North Dakota. As of the 2020 census, the population was 184,525. Cass County is the most populous county in North Dakota, accounting for nearly 24% of the state's population. The county seat is Fargo, the state's most populous city. The county is named for George Washington Cass, president of the Northern Pacific Railway from 1872 to 1875. It is the only Cass County in the United States that is not named after Lewis Cass.

Fargo is a city in and the county seat of Cass County, North Dakota, United States. According to the 2020 census, its population was 125,990, making it the most populous city in the state and the 216th most populous city in the United States. Fargo, along with its twin city of Moorhead, Minnesota, and the adjacent cities of West Fargo, North Dakota and Dilworth, Minnesota, form the core of the Fargo–Moorhead, ND-MN Metropolitan Statistical Area (MSA). The MSA had a population of 248,591 in 2020.