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

This species was described from Flinders, Westernport Bay, Victoria, Australia.

[2] This dorid nudibranch is bright orange to reddish-orange in colour, and the dorsum is covered with caryophyllidia.

[1] This nudibranch is found on a colony of the red sponge, Clathria cactiformis (family Microcionidae) on which it presumably feeds.

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