Goniobranchus geminus, also known as the gem sea slug,[3] is a species of very colourful sea slug, a dorid nudibranch, a marine gastropod mollusc in the family Chromodorididae.

[2] Until 2012 this species was known as Chromodoris geminus but was moved to the genus Goniobranchus as a result of a molecular (DNA) study.

[4] Goniobranchus geminus can reach a maximum size of 5 cm in length.

[6] The background coloration from the back is light brown to yellowish speckled with mauve ocelli circled with a white margin.

The rhinophores are yellow or purple and laminated, gills are white outlined with beige.