Waterloo is a city in and the county seat of Black Hawk County, Iowa, United States. It is the eighth-most populous city in Iowa, with a population of 67,314 at the 2020 census. Waterloo comprises a twin conurbation with neighboring municipality Cedar Falls, and is the larger of the two cities. The Waterloo – Cedar Falls metropolitan area has an estimated 170,000 residents. Waterloo is bisected into eastern and western halves by the Cedar River flowing southeastward.