Atrophacanthus is a monospecific genus of marine ray-finned fish belonging to the family Triacanthodidae, the spikefishes.

The only species in the genus is Atrophacanthus japonicus, the upward-mouth spikefish, which is found in the Indo-West Pacific region.

A. danae was subsequently shown to be a synonym of Tydemania japonica which had been described in 1941 by the Japanese ichthyologist Toshiji Kamohara with its type locality given as Shikoku in the Kōchi Prefecture, Japan.

[2] This taxon is classified within the subfamily Triacanthodinae of the spikefish family Triacanthodidae, within the suborder Triacanthoidei of the order Tetraodontiformes in the 5th edition of Fishes of the World.

[6] Atrophacanthus is carnivorous and the stomach contents of two juveniles collected near Guam consisted of formanifera and thecosome pteropods.