Seneca County is a county located in the U.S. state of New York. As of the 2010 census, the population was 35,251. The primary county seat is Waterloo, moved from the original county seat of Ovid in 1819. It became a two-shire county in 1822, using both locations as county seats although the majority of Seneca County administrative offices are in Waterloo. Therefore, most political sources only list Waterloo as the county seat. The county's name is from the Iroquois (Seneca) that occupied part of the region.