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

[2] This species has previously been confused with Phyllidia ocellata.

[3] This species was described from the Red Sea.

It appears to be confined to the western Indian Ocean including the Red Sea, the African coast and the Island of Réunion.

[4] This species feeds on a sponge.