Coccosteus ("Seed Bone") is an extinct genus of arthrodire placoderm. Its fossils have been found throughout Europe and North America. The majority of these have been found in freshwater sediments, though, such a large range suggests that they may have been able to enter saltwater. The largest specimens were about 40 centimetres (16 in), although the average length was 20 to 24 centimetres (7.9 to 9.4 in)

It was very closely related to Dunkleosteus, A much larger fish that would probably eat Coccosteus. It would also fall prey to amphibians and other large fish like Hyneria and sharks like Stethacanthus, also known as the ironing board shark.