Rostanga risbeci, is a species of sea slug, a dorid nudibranch, a marine gastropod mollusc in the family Discodorididae.

This species was described from Aitsu, Amakusa, Japan with additional specimens from Sagami Bay and the Echizen coast.

[1][2] Rostanga risbeci is a purple-black or black dorid nudibranch with the dorsum covered with caryophyllidia; in general it is very similar in shape, but not in colour, to other species of Rostanga.

[1] This nudibranch is reported to feed on the black sponge Halichondria okadai - family Halichondriidae.

[1] Most species of Rostanga feed on sponges of the family Microcionidae.