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. Therefore, many political sources only list Waterloo as the county seat.

Seneca County is located in the U.S. state of New York. As of the 2020 census, the population was 33,814. The primary county seat is Waterloo, moved there from the original county seat of Ovid in 1819. It became a two-shire county in 1822, which currently remains in effect and uses both locations as county seats although the majority of Seneca County administrative offices are located in Waterloo. Therefore, most political sources list only Waterloo as the county seat. The county's name comes from the Seneca Nation of the Haudenosaunee (Iroquois), who occupied part of the region. The county is part of the Finger Lakes region of the state.