Hymenocephalus lethonemus is a species of rattail. It occurs at depths of up to 485 m in the waters off southern Japan, the Philippines and northern Taiwan.

This is a small, slender rattail with a total length of up to 14 cm. It has a fairly long, sharp snout, small eyes, a large mouth and no chin barbel. There is a long bioluminescent organ with two external lenses just in front of the anus.