Okenia hallucigenia is a species of sea slug, specifically a dorid nudibranch, a marine gastropod mollusc in the family Goniodorididae.

[2] This species was named because of its resemblance to the shape of the Cambrian fossil Hallucigenia.

[3] This Okenia has a narrow body and long lateral papillae.

This sea slug is very similar to Okenia stellata, and can be found with that species on the same food.

There are two other species from southern Japan, the Marshall Islands, the Philippines and Indonesia which are also similar to this species, and those are Okenia nakamotoensis and Okenia kondoi.