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.