Wahpeton, North Dakota. Wahpeton (/ˈwɑːpᵻtən/ WAH-pə-tən) is a city in Richland County, in southeastern North Dakota, United States. It is located along the Bois de Sioux River at its confluence with the Otter Tail River, forming the Red River of the North. Wahpeton is the county seat of Richland County.