| Named By: | Agassiz in 1843 |
| Time Period: | Early Carboniferous-Late Permian |
| Location: | Potentially worldwide with fossils being found in Australia, England, Germany, Greenland, Jan Mayan, Russia, Scotland, Svalbard and the USA including the states of Illinois and Kansas |
| Size: | About 18 centimetres long |
| Diet: | Planktonic feeder |
| Fossil(s): | Multiple specimens |
| Classification: | | Chordata | Actinopterygii | Palaeonisciformes | |
| Also known as: | | Platysomus parvus | Stromateus gibbosus | |
Platysomus is an extinct genus of ray-finned fish that lived in the Carboniferous and Permian periods. Fossils have been found worldwide.
Platysomus was about 18 centimetres (7.1 in) long, and shaped similarly to the discus fish, having the same flattened body and alongated dorsal and anal fins. Its jaws were placed vertically under the braincase, giving it a wide gape. Platysomus is thought to have fed on plankton, and lived in both fresh and salt water.