High Quality Content by WIKIPEDIA articles! High Quality Content by WIKIPEDIA articles! Flat Rock is an unincorporated community in northeastern Thompson Township, Seneca County, Ohio, United States. Although it is unincorporated, it has a post office, with the ZIP code of 44828. Thompson Township is one of the fifteen townships of Seneca County, Ohio, United States. The 2000 census found 1,422 people in the township.