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. It is the home of Heidelberg University and Tiffin University. At one time the city was noted as a glass and porcelain manufacturing center. The National Arbor Day Foundation has designated Tiffin as a Tree City USA.