A slender, black shark reaching 54 cm (21 in) in length, the viper dogfish can be recognized by its narrow, triangular jaws and well-spaced, fang-like teeth.

The skeletal and muscular structure of its head shows unique features that support this feeding mechanism, which is unlike that of other dogfish sharks.

The shark was described as a new species and genus by University of Tokyo researchers Kenji Mochizuki and Fumio Ohe in a 1990 article for the Japanese Journal of Ichthyology.

They gave it the name Trigonognathus kabeyai; the generic name is derived from the Greek trigonon ("triangle") and gnathus ("jaw"),[citation needed] while the specific name honors Hiromichi Kabeya, the captain of the Seiryo-Maru.

[3] Mochizuki and Ohe originally assigned the viper dogfish to the family Squalidae, which at the time was used for all members of the order Squaliformes aside from the bramble and rough sharks.

[3] In a 1992 morphological study, Shigeru Shirai and Osamu Okamura placed this species in the squalid subfamily Etmopterinae, which most taxonomists now recognize as the separate family Etmopteridae.

[5][6][7] Based on molecular clock estimation, Trigonognathus is thought to have originated around 41 million years ago during the Middle Eocene, as part of a larger evolutionary radiation of etmopterid genera.

[5] The genus is represented in this time period by the extinct species T. virginiae, whose fossilized teeth have been recovered from Lutetian age (47.8–41.3 Mya) strata in Landes, southwestern France.

[8] Fossil teeth virtually identical to those of the modern viper dogfish are known from the Cubagua Formation in northeastern Venezuela, which dates to the Late Miocene and Early Pliocene (11.6–3.6 Mya).

[2][3] Most specimens of the viper dogfish have been collected from a relatively small area of the northwestern Pacific Ocean off the Kii Peninsula in Japan.

This pattern suggests the viper dogfish performs a diel vertical migration, spending the day in deeper water and rising towards the surface at night; such daily movement may relate to feeding.

[2] The viper dogfish feeds primarily on bony fishes, notably lanternfishes in the genera Benthosema and Diaphus, and also takes crustaceans.

Like other members of its family, this species is viviparous, giving birth to live offspring, with the developing embryos sustained to term by yolk.