Seneca County is a county located in the northwestern part of the U.S. state of Ohio. As of the 2020 census, the population was 55,069. Its county seat is Tiffin. The county was created in 1820 and organized in 1824. It is named for the Seneca Indians, the westernmost nation of the Iroquois Confederacy. This people were based in present-day New York but had territory extending into Pennsylvania and Ohio.