Tiffin is a city in and the county seat of Seneca County, Ohio, United States. Developed along the Sandusky River, Tiffin is located about 5…Tiffin is a city in and the county seat of Seneca County, Ohio, United States. Developed along the Sandusky River, Tiffin is located about 55 miles southeast of Toledo. The population was 17,953 at the 2020 census.