Genus: Megaptergius KONISHI, OHARA, MISAKI, MATSUOKA, STREET & CALDWELL, 202
Etymology: Greek, megas, "large" and Greek, pterygion, "wing", in reference to the characteristically large, wing-shpaed flippers of this new mosasaur.

Species: wakayamaensis KONISHI, OHARA, MISAKI, MATSUOKA, STREET & CALDWELL, 2023
Etymology: In reference to the Wakayama Prefecture, south-western Japan, where the holotype was collected and is curated.

Holotype: WMNH-Ge-1140240002

Locality: About 350 m south of the peak of Mt Toyajo, Wakayama Prefecture, Kansi Region,Honshu Island, south-western Japan.

Horizon: Near the top of the Haseegawa Muddy Sandstone Member, Toyajo Formation.

Biostratigraphy:

Age: Late Campanian-Early Maastrichtian Stage, uppermost Senonian subepoch, Gulf Epoch, Late Cretaceous.

Material: Nearly complete skull and incomplete skeleton.