Rostanga phepha, is a species of sea slug, a dorid nudibranch, a marine gastropod mollusc in the family Discodorididae.
This species has so far only been found around the southern African coast, from the Atlantic side of the Cape Peninsula. This nudibranch appears to be endemic to South Africa.
Distinctively different to other Rostanga species, this animal has a white notum with elongate, scattered, purple-brown spots.