Phyllidia varicosa is a species of sea slug, a dorid nudibranch, a shell-less marine gastropod mollusc in the family Phyllidiidae.

It can be distinguished by its numerous (3 to 6), longitudinal, tuberculate notal ridges.

[4] Juveniles of the sea cucumber, Pearsonothuria graeffei are brightly coloured and closely resemble Phyllidia varicosa.

They are white and blue or black, with a few large, yellow, thornlike projections.

These bright colours warn predators of the toxicity of the nudibranch, and this mimicry on the part of the sea cucumber species serves to protect it also.