Coccosteus is an extinct genus of Placoderms (class Placodermi) lived in the Middle Devonian ^{3), 4)}. Bothriolepis and Dunkleosteus are also Placoderms, especially the later and Coccosteus are classified to the same order Arthrodira (meaning "jointed neck"), suborder Brachythoraci (meaning "short thorax") ⁴⁾.

The type species Coccosteus cuspidatus is about 20 cm in length, that is, not a large fish ³⁾.

Although Coccosteus represented the early Brachythoraci ⁴⁾, it could open its mouth wide and was presumably able to suck large amount of water at once, including small animals and sedimentary mud ³⁾. But, it had no large fearsome "front tooth" like that of Dunkelosteus (or Dinichthys) ²⁾. (The "tooth" of Placoderms is not a true tooth, but a part of bone. ⁴⁾)

Specimens of Coccosteus have been found in the sedimentary layers formed in the non-marine environments such as lakes or rivers (e.g.the Scottish Middle Devonian Old Red Sandstone) ^{1), 4)}, suggesting Coccosteus lived in the freshwater environments ³⁾.