Waterloo is a village and primary county seat of Seneca County, New York, United States. The population was 5,171 at the 2010 census and is now the most populated village in Seneca County. The village is named after Waterloo, Belgium, where Napoleon was defeated. It is the primary county seat of Seneca County, with the other being Ovid as part of a two-shire system established in 1822. Most of the county administrative offices are located in the village.