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.