Holoptychius (from Greek: όλος holos, 'whole' and Greek: πτυχή ptyche 'fold') is an extinct genus of porolepiform lobe-finned fish from the Middle Devonian to Carboniferous (Mississippian) periods. It is known from fossils worldwide. The genus was first described by Louis Agassiz in 1839.

Holoptychius was a streamlined predator about 50 centimetres (20 in) long (though largest specimen could grow up to 2.5 metres (8.2 ft)), which fed on other bony fish. Its rounded scales and body form indicate that it could have swum quickly through the water to catch prey. Similar to other rhipidistians, it had fang-like teeth on its palate in addition to smaller teeth on the jaws. Its asymmetrical tail sported a caudal fin on its lower end. To compensate for the downward push caused by this fin placement, the pectoral fins of Holoptychius were placed high on the body.

Of the genus Holoptychius the following species have been described:

Fossils of Holoptychius have been found in the Mississippian of the United Kingdom and the Devonian of Belgium, Colombia (Cuche Formation, Boyacá), Norway, Canada, the Russian Federation, and the United States (Pennsylvania).

Isotopic analysis of Holoptychius remains from Greenland reveals that it was likely a euryhaline organism.

Media related to Holoptychius at Wikimedia Commons