Goniobranchus geometricus is a species of colourful sea slug, a dorid nudibranch, a marine gastropod mollusc in the family Chromodorididae.

[4][5] 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.

[4][6][7] This animal feeds on the sponge Chelonaplysilla violacea.