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.

A county seat is an administrative center, seat of government, or capital city of a county or civil parish. The term is in use in five countries: Canada, China, Hungary, Romania, and the United States. An equivalent term, shire town, is used in the U.S. state of Vermont and in several other English-speaking jurisdictions. County towns usually serve a similar function in Ireland, Jamaica, and the United Kingdom.