Flat Rock is a census-designated place in northeastern Thompson Township, Seneca County, Ohio, United States.

Flat Rock was originally called Lewisville, and under the latter name was platted in 1841.

[4] A post office called Flat Rock has been in operation since 1846.

[5] The present name comes from a flat layer of rock covering an artesian aquifer.

This Seneca County, Ohio state location article is a stub.