Waterloo is a city in and the county seat of Black Hawk County, Iowa, United States. As of the 2010 United States Census the population decreased by 0.5% to 68,406; the 2014 Census estimates the population at 68,364, making it the sixth-largest city in the state. The city is part of the Waterloo ? Cedar Falls Metropolitan Statistical Area, and is the more populous of the two cities.