Goniobranchus geometricus is a species of colourful sea slug, a dorid nudibranch, a marine gastropod mollusc in the family Chromodorididae.
Goniobranchus geometricus is a small nudibranch which may grow to a total length of 35mm. It is variably coloured, with the ground colour ranging from a creamy brown to purple, The mantle has numerous cream-coloured bumps and has a white margin. The gills and rhinophores are white to greenish. It is very similar to Goniobranchus conchyliatus but has green-grey rhinophores and gills whilst G. conchyliatus has red ones.
This species was described from New Caledonia. It is found throughout the Indian Ocean as well as the Western Pacific Ocean from the intertidal down to 30 m (98 ft).
This animal feeds on the sponge Chelonaplysilla violacea.
