Goniobranchus obsoletus is a species of colourful sea slug, a dorid nudibranch, a marine gastropod mollusc in the family Chromodorididae.
Goniobranchus obsoletus is a chromodorid nudibranch with a mostly white mantle and an orange mantle edge. There is an irregular band of blue-black just inside the orange margin and the mantle is rugose with an orange-brown reticulation between the raised pustules. The rhinophores and gills are translucent brown with white markings.
This species was described from the Red Sea. It has also been reported from the Persian Gulf.
