[3] This species is currently classified in the genus Leptochilichthys within the family Alepocephalidae in the order Alepocephaliformes.

The "thin lip" part is thought to refer to the slender maxilla and intermaxilla of the type species of the genus, L. agassizii.

The specific name, pinguis, means "fat", a reference to fold or cushion of adipose tissue that runs along the back in front of the dorsal fin.

[5] Vaillant's smooth-head has an elongated and streamlined body with a smooth head which has no obvious ridges on it.

The skin is smooth and the overall colour is dark brown to grey, paler ventrally.