| Named By: | Brian Choo, Min Zhu, Wenjin Zhao, Liaotao Jia & You'an Zhu in 2014 |
| Time Period: | Late Silurian, 423 Ma |
| Location: | China - Kuanti Formation |
| Size: | Reconstructed jaw perceived to be 16 centimetres long. Full length estimated to be anywhere up to 1 meter long for the holotype |
| Diet: | Carnivore |
| Fossil(s): | Partial upper and lower jaw bones from three individuals |
| Classification: | | Chordata | Sarcopterygii | |
Megamastax is a genus of lobe-finned fish which lived during the late Silurian period, about 423 million years ago. The generic name of Megamastax is Greek for 'big mouth' coming from megalos (big) and mastax (mouth). Fossils of Megamastax were found at Kuanti Formation in Qujing, Yunnan, China. Before the discovery of Megamastax, it was thought that jawed vertebrates (gnathostomes) were limited in size and variation before the Devonian period. Megamastax is known only from jaw bones and it is estimated that it reached about 1 m long.