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

Its distribution includes also Rio de Janeiro State, southeastern Brazil.

[2][3] This doric nudibranch is bright orange to reddish orange in colour, and the dorsum is covered with caryophyllidia; it is very similar to other species of Rostanga.

The colour is very uniform, but the rhinophores have white tips.

[5] The prey of Rostanga byga includes the sponge Mycale microsigmatosa.