Dinichthys herzeri is an extinct, giant, marine arthrodire placoderm from the Late Devonian (Famennian stage) of Ohio and Tennessee. It was comparable in size, shape, and ecological role to the better-known Dunkleosteus. Originally described in 1868 by John Newberry on the basis of an incomplete skull roof and mandibles (AMNH 81), this species remains imperfectly known to this day.